compositor: Remove wl_output-global on unplug

Or we would segfault when a client binds to a output thats already
destroyed.
diff --git a/src/compositor.h b/src/compositor.h
index 1b37356..9022313 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -73,6 +73,7 @@
 
 struct weston_output {
 	struct wl_list link;
+	struct wl_global *global;
 	struct weston_compositor *compositor;
 	struct weston_matrix matrix;
 	struct wl_list frame_callback_list;