sessiond: make thread_consumer_management function static

Fixes:

      CC       manage-consumer.o
    /home/smarchi/src/lttng-tools/src/bin/lttng-sessiond/manage-consumer.c:64:7: error: no previous declaration for ‘thread_consumer_management’ [-Werror=missing-declarations]
     void *thread_consumer_management(void *data)
           ^~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: Id28f40244e12a618749cc618466fc31d6d773378
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>