SessionMode: add polkit to gdm session

It's needed to shutdown when other users are logged in or inhibitors are
active.

https://bugzilla.gnome.org/show_bug.cgi?id=683400

js/ui/sessionMode.js

@@ -33,6 +33,7 @@ const _modes = {
         allowKeybindingsWhenModal: true,
         isGreeter: true,
         unlockDialog: imports.gdm.loginDialog.LoginDialog,
+        components: ['polkitAgent'],
         panel: {
             left: [],
             center: ['dateMenu'],