gdm: pull async fix from upstream

This prevents a race at start up that could prevent the user
from getting loaded properly.

In the near future we need to drop the gdm user code entirely
and switch to using the accountsservice library.

src/gdmuser/gdm-user-manager.c

@@ -1199,6 +1199,8 @@ on_list_cached_users_finished (DBusGProxy     *proxy,
                 return;
         }
 
+        maybe_set_is_loaded (manager);
+
         g_ptr_array_foreach (paths, (GFunc)add_new_user_for_object_path, manager);
 
         g_ptr_array_foreach (paths, (GFunc)g_free, NULL);