loginDialog: Add missing return value

_loadUserList() may be used as idle handler, so we should explicitly
return a boolean.

https://bugzilla.gnome.org/show_bug.cgi?id=725905

js/gdm/loginDialog.js

@@ -861,6 +861,8 @@ const LoginDialog = new Lang.Class({
                                   Lang.bind(this, function(userManager, user) {
                                       this._userList.removeUser(user);
                                   }));
+
+        return GLib.SOURCE_REMOVE;
     },
 
     open: function() {