sessiond: make disable_context static

sessiond: make disable_context static

Fixes:

      CC       agent.o
    /home/smarchi/src/lttng-tools/src/bin/lttng-sessiond/agent.c:833:5: error: no previous declaration for ‘disable_context’ [-Werror=missing-declarations]
     int disable_context(struct agent_app_ctx *ctx, enum lttng_domain_type domain)
         ^~~~~~~~~~~~~~~

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I3f8eb7e658e558f7082781c95b4200607e5dad6f
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>