cleanup: Mark globals used from other modules as exported

eslint cannot figure out that those symbols are used from other modules
via imports, so they trigger unused-variable errors. To fix, explicitly
mark those symbols as exported.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/627

js/ui/switchMonitor.js

@@ -1,4 +1,5 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+/* exported SwitchMonitorPopup */
 
 const { Clutter, GObject, Meta, St } = imports.gi;