agent: Fix warning about required glib version

agent: Fix warning about required glib version

../agent/discovery.c: In function ‘on_refresh_remove_timeout’:
../agent/discovery.c:203:13: warning: Not available before 2.60
  203 |       G_GNUC_FALLTHROUGH;
      |             ^~~~~~~~~~~~~

../agent/conncheck.c: In function ‘priv_turn_allocate_refresh_retransmissions_tick_agent_locked’:
../agent/conncheck.c:1656:13: warning: Not available before 2.60
 1656 |       G_GNUC_FALLTHROUGH;
      |             ^~~~~~~~~~~~~

The comment /* fall through */ is enough to cause GCC and Clang to not
emit the implicit-fallthrough warning.