Fix objects not appearing in Outliner after dragging in from Asset Browser

When dragging in an object from an external asset library from the Asset
Browser, the Outliner wouldn't update.

source/blender/editors/space_view3d/space_view3d.c

@@ -747,6 +747,8 @@ static void view3d_ob_drop_copy_external_asset(wmDrag *drag, wmDropBox *drop)
 
   ID *id = WM_drag_asset_id_import(asset_drag, FILE_AUTOSELECT);
 
+  WM_event_add_notifier(C, NC_SCENE | ND_LAYER_CONTENT, scene);
+
   RNA_string_set(drop->ptr, "name", id->name + 2);
 
   Base *base = BKE_view_layer_base_find(view_layer, (Object *)id);