Use G_GNUC_FALLTHROUGH instead of comments

Get the same task done in a bit more C:y way.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/998

src/backends/native/meta-barrier-native.c

@@ -367,7 +367,7 @@ emit_barrier_event (MetaBarrierImplNative *self,
     case META_BARRIER_STATE_LEFT:
       self->state = META_BARRIER_STATE_ACTIVE;
 
-      /* Intentional fall-through. */
+      G_GNUC_FALLTHROUGH;
     case META_BARRIER_STATE_HELD:
       event->dt = time - self->last_event_time;