loginDialog: Make notListedButton accessible

Without this a screen reader just calls the notListedButton a
"push button" instead of also including the "Not listed?" text.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1567>

js/gdm/loginDialog.js

@@ -464,6 +464,7 @@ var LoginDialog = GObject.registerClass({
             child: notListedLabel,
             reactive: true,
             x_align: Clutter.ActorAlign.START,
+            label_actor: notListedLabel,
         });
 
         this._notListedButton.connect('clicked', this._hideUserListAskForUsernameAndBeginVerification.bind(this));