lttng-ctl: add local declarations for obsolete functions in lttng-ctl.c

The functions concerned by this change are obsolete, but kept to avoid
breaking the ABI.  This patch adds local declaration for these functions
to silence the following errors, when building with
-Wmissing-declarations.

      CC       lttng-ctl.lo
    /home/smarchi/src/lttng-tools/src/lib/lttng-ctl/lttng-ctl.c:2815:5: error: no previous declaration for ‘lttng_enable_consumer’ [-Werror=missing-declarations]
     int lttng_enable_consumer(struct lttng_handle *handle)
         ^~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/src/lib/lttng-ctl/lttng-ctl.c:2823:5: error: no previous declaration for ‘lttng_disable_consumer’ [-Werror=missing-declarations]
     int lttng_disable_consumer(struct lttng_handle *handle)
         ^~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/src/lib/lttng-ctl/lttng-ctl.c:2831:5: error: no previous declaration for ‘_lttng_create_session_ext’ [-Werror=missing-declarations]
     int _lttng_create_session_ext(const char *name, const char *url,
         ^~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I49750d7646d5823461933c13a9ec4ccef905921c
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>