Fix: sessiond: make the --without-lttng-ust version of launch_application_notificatio...

When building with --without-lttng-ust, a simple version of
launch_application_notification_thread, implemented in the header file,
is used.  We get this warning:

      CC       main.o
    In file included from /home/simark/src/lttng-tools/src/bin/lttng-sessiond/main.c:61:
    /home/simark/src/lttng-tools/src/bin/lttng-sessiond/notify-apps.h:17:6: error: no previous prototype for ‘launch_application_notification_thread’ [-Werror=missing-prototypes]
       17 | bool launch_application_notification_thread(int apps_cmd_notify_pipe_read_fd)
          |      ^~~~~~~

Make the function `static inline` to avoid that.  The `inline` is not
strictly required here, but if that header ended up included by some
other source file that didn't use
launch_application_notification_thread, we would get a -Wunused-function
warning.  The `inline` avoids that.

Change-Id: I19605e0594af0d7997951def2da3a6313bf65e11
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>