js: Fix util_translate_time_string() usage

util_translate_time_string() was called to use LC_TIME to translate
strings, but those strings were not marked as to be translated.

https://bugzilla.gnome.org/show_bug.cgi?id=739822

js/ui/screenShield.js

@@ -85,7 +85,7 @@ const Clock = new Lang.Class({
         let date = new Date();
         /* Translators: This is a time format for a date in
            long format */
-        let dateFormat = Shell.util_translate_time_string("%A, %B %d");
+        let dateFormat = Shell.util_translate_time_string(N_("%A, %B %d"));
         this._date.text = date.toLocaleFormat(dateFormat);
     },