screen/wayland: init all known fields to NULL

Fixes a missing initializer warnings. This is similar to how it's done
for wl_pointer_listener.

modules/access/screen/wayland.c

@@ -153,6 +153,12 @@ const struct wl_output_listener output_cbs =
     output_mode_cb,
     NULL,
     NULL,
+#ifdef WL_OUTPUT_NAME_SINCE_VERSION
+    NULL,
+#endif
+#ifdef WL_OUTPUT_DESCRIPTION_SINCE_VERSION
+    NULL,
+#endif
 };
 
 static void screenshooter_done_cb(void *data,