compositor: fix destroy_frame_callback()

Pass the correct pointer to free().

This is just a cosmetic change, because 'resource' happens to be the
first member in wlsc_frame_callback.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

compositor/compositor.c

@@ -1133,7 +1133,7 @@ destroy_frame_callback(struct wl_resource *resource)
 	struct wlsc_frame_callback *cb = resource->data;
 
 	wl_list_remove(&cb->link);
-	free(resource);
+	free(cb);
 }
 
 static void