Added userdata to wlr_output_layout

Needed for https://github.com/swaywm/wlroots-rs/issues/131

include/wlr/types/wlr_output_layout.h

@@ -17,6 +17,8 @@ struct wlr_output_layout {
 		struct wl_signal change;
 		struct wl_signal destroy;
 	} events;
+
+	void *data;
 };
 
 struct wlr_output_layout_output_state;