Move 'use strict' to file level

Change-Id: I1f0dae4aa168e77c112bcd3688c98b84e4ce5f36
diff --git a/dev/js/src/loc/dereko.js b/dev/js/src/loc/dereko.js
index 9f08470..2cdae7d 100644
--- a/dev/js/src/loc/dereko.js
+++ b/dev/js/src/loc/dereko.js
@@ -4,6 +4,8 @@
  * @author Helge Stallkamp
  */
 
+"use strict";
+
 define(['vc', 'vc/doc'], function (vcClass, docClass) {
   const loc = KorAP.Locale;