Fix layer shell subsurface use after free

This occurs when a layer surface is destroyed , prompting an unmap event
for all its children subsurfaces (if it has any). Since this happens after
the parent surface is destroyed, any pointers to it will become dangling.

include/sway/layers.h

@@ -48,6 +48,7 @@ struct sway_layer_subsurface {
 	struct wl_listener unmap;
 	struct wl_listener destroy;
 	struct wl_listener commit;
+	struct wl_listener layer_surface_destroy;
 };
 
 struct sway_output;