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/misc/systemActions.js

@@ -1,3 +1,4 @@
+/* exported getDefault */
 const { AccountsService, Clutter, Gdm, Gio, GLib, GObject, Meta } = imports.gi;
 
 const GnomeSession = imports.misc.gnomeSession;