sessiond: make functions static in ust-registry.c

Fixes:

      CC       ust-registry.o
    /home/smarchi/src/lttng-tools/src/bin/lttng-sessiond/ust-registry.c:553:2: error: no previous declaration for ‘ust_registry_lookup_enum’ [-Werror=missing-declarations]
      ust_registry_lookup_enum(struct ust_registry_session *session,
      ^~~~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/src/bin/lttng-sessiond/ust-registry.c:684:6: error: no previous declaration for ‘ust_registry_destroy_enum’ [-Werror=missing-declarations]
     void ust_registry_destroy_enum(struct ust_registry_session *reg_session,
          ^~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: Ibaabd16a75f0b7b13a641a571d26ca64aec5317b
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>