Bug 1621445 - Part 8: Fix missing declaration. r=bustage-fix

suite/components/nsSuiteGlue.js

@@ -535,7 +535,7 @@ SuiteGlue.prototype = {
                       "downloads", "cookies", "cache", "sessions",
                       "offlineApps" ];
 
-      for (pref of prefs) {
+      for (let pref of prefs) {
         try {
           let prefOld = "privacy.item." + pref;