js: Avoid double declarations with let

The following code is a syntax error in ES6:

    let a = 'something';
    let a = 'other thing';

Previously GJS would silently accept this code, but in the next release the
SpiderMonkey JS engine will be more ES6-compliant.

https://bugzilla.gnome.org/show_bug.cgi?id=778425

js/ui/notificationDaemon.js

@@ -186,7 +186,7 @@ const FdoNotificationDaemon = new Lang.Class({
             return source;
         }
 
-        let source = new FdoNotificationDaemonSource(title, pid, sender, ndata ? ndata.hints['desktop-entry'] : null);
+        source = new FdoNotificationDaemonSource(title, pid, sender, ndata ? ndata.hints['desktop-entry'] : null);
 
         this._sources.push(source);
         source.connect('destroy', Lang.bind(this, function() {