telepathy-client: Replace shell_util_new_from_string

The function has been removed in commit 786cfbd397, but one user
was overlooked when replacing it.

https://bugzilla.gnome.org/show_bug.cgi?id=661231

js/ui/telepathyClient.js

@@ -1297,7 +1297,7 @@ MultiNotificationSource.prototype = {
     },
 
     createNotificationIcon: function() {
-        return new St.Icon({ gicon: Shell.util_icon_from_string(this._icon),
+        return new St.Icon({ gicon: Gio.icon_new_for_string(this._icon),
                              icon_type: St.IconType.FULLCOLOR,
                              icon_size: this.ICON_SIZE });
     }