Add a "falls through" comment to make gcc happy.

src/or/circuitbuild.c

@@ -2288,6 +2288,7 @@ choose_good_exit_server(origin_circuit_t *circ, int need_uptime,
       /* For these three, we want to pick the exit like a middle hop,
        * since it should be random. */
       tor_assert_nonfatal(is_internal);
+      /* Falls through */
     case CIRCUIT_PURPOSE_C_GENERAL:
       if (is_internal) /* pick it like a middle hop */
         return router_choose_random_node(NULL, options->ExcludeNodes, flags);