authPrompt: Make actor reactive

The actor is non-reactive, but has a key event handler to handle
the Esc key on the auth prompt. Mark this actor as reactive, so
it can handle the events.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2045>

js/gdm/authPrompt.js

@@ -55,6 +55,7 @@ var AuthPrompt = GObject.registerClass({
             vertical: true,
             x_expand: true,
             x_align: Clutter.ActorAlign.CENTER,
+            reactive: true,
         });
 
         this.verificationStatus = AuthPromptStatus.NOT_VERIFYING;