Define classes with 'var' instead of 'const'

Any symbols (including class properties) that should be visible
outside the module it's defined in need to be defined as global.
For now gjs still allows the access for 'const', but get rid of
the warnings spill now by changing it.

https://bugzilla.gnome.org/show_bug.cgi?id=785084

js/gdm/authPrompt.js

@@ -38,7 +38,7 @@ const BeginRequestType = {
     DONT_PROVIDE_USERNAME: 1
 };
 
-const AuthPrompt = new Lang.Class({
+var AuthPrompt = new Lang.Class({
     Name: 'AuthPrompt',
 
     _init: function(gdmClient, mode) {