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/barLevel.js

@@ -1,4 +1,5 @@
 /* -*- mode: js2; js2-basic-offset: 4; indent-tabs-mode: nil -*- */
+/* exported BarLevel */
 
 const { Atk, Clutter, St } = imports.gi;
 const Signals = imports.signals;