sessiond: make ltt_sessions_ht_alloc static

sessiond: make ltt_sessions_ht_alloc static

Fixes:

      CC       session.o
    /home/smarchi/src/lttng-tools/src/bin/lttng-sessiond/session.c:305:5: error: no previous declaration for ‘ltt_sessions_ht_alloc’ [-Werror=missing-declarations]
     int ltt_sessions_ht_alloc(void)
         ^~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: Iaf4ca32c1f6ca01fa7ad4a1bfc7edd81a90e6adc
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>