Fix: build failures when `--without-lttng-ust`

Fix: build failures when `--without-lttng-ust`

The `--without-lttng-ust` configure option makes the build to fail due
to 2 bugs
1. Missing object file for session unit tests causes the following errors:
    CCLD     test_session
  ../../src/bin/lttng-sessiond/kernel.o: In function `kernel_track_id':
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/kernel.c:712: undefined reference to `lttng_tracker_id_lookup_string'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/kernel.c:723: undefined reference to `lttng_tracker_id_get_list'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/kernel.c:729: undefined reference to `lttng_tracker_list_add'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/kernel.c:810: undefined reference to `lttng_tracker_id_set_list'
  ../../src/bin/lttng-sessiond/kernel.o: In function `kernel_untrack_id':
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/kernel.c:826: undefined reference to `lttng_tracker_id_lookup_string'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/kernel.c:836: undefined reference to `lttng_tracker_id_get_list'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/kernel.c:841: undefined reference to `lttng_tracker_list_remove'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/kernel.c:925: undefined reference to `lttng_tracker_id_set_list'
  ../../src/bin/lttng-sessiond/kernel.o: In function `kernel_list_tracker_ids':
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/kernel.c:949: undefined reference to `lttng_tracker_id_get_list'
  ../../src/bin/lttng-sessiond/trace-kernel.o: In function `trace_kernel_create_session':
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/trace-kernel.c:167: undefined reference to `lttng_tracker_list_create'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/trace-kernel.c:171: undefined reference to `lttng_tracker_list_create'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/trace-kernel.c:175: undefined reference to `lttng_tracker_list_create'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/trace-kernel.c:179: undefined reference to `lttng_tracker_list_create'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/trace-kernel.c:183: undefined reference to `lttng_tracker_list_create'
  ../../src/bin/lttng-sessiond/trace-kernel.o:/home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/trace-kernel.c:187: more undefined references to `lttng_tracker_list_create' follow
  ../../src/bin/lttng-sessiond/trace-kernel.o: In function `trace_kernel_create_session':
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/trace-kernel.c:199: undefined reference to `lttng_tracker_list_destroy'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/trace-kernel.c:200: undefined reference to `lttng_tracker_list_destroy'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/trace-kernel.c:201: undefined reference to `lttng_tracker_list_destroy'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/trace-kernel.c:202: undefined reference to `lttng_tracker_list_destroy'
  /home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/trace-kernel.c:203: undefined reference to `lttng_tracker_list_destroy'
  ../../src/bin/lttng-sessiond/trace-kernel.o:/home/frdeso/projets/lttng/tools/src/bin/lttng-sessiond/trace-kernel.c:204: more undefined references to `lttng_tracker_list_destroy' follow
  collect2: error: ld returned 1 exit status
  Makefile:1062: recipe for target 'test_session' failed
  make[1]: *** [test_session] Error 1
  make[1]: Leaving directory '/home/frdeso/projets/lttng/tools/tests/unit'
  Makefile:1166: recipe for target 'all-recursive' failed
  make: *** [all-recursive] Error 1

2. An extra colon at the end of the `trace_ust_track_id()` stub
function gives this error:
    CC       utils.o
  In file included from ust-app.h:26:0,
                   from lttng-sessiond.h:31,
                   from utils.c:27:
  trace-ust.h:339:1: error: expected identifier or '(' before '{' token
   {
   ^
  trace-ust.h:336:19: warning: 'trace_ust_track_id' declared 'static' but never defined [-Wunused-function]
   static inline int trace_ust_track_id(enum lttng_tracker_type tracker_type,
                     ^~~~~~~~~~~~~~~~~~
  Makefile:729: recipe for target 'utils.o' failed

Both bugs are trivial to fix.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I66c57d206ad0142db057443374d28bbcf16c02f7
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>