Bug 1403345, part 1 - Add missing include to nsReferencedElement.h. r=longsonr

MozReview-Commit-ID: KDmGJJ33bfN

dom/base/nsReferencedElement.cpp

@@ -5,6 +5,8 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "nsReferencedElement.h"
+
+#include "mozilla/Encoding.h"
 #include "nsContentUtils.h"
 #include "nsIURI.h"
 #include "nsBindingManager.h"