Add magic emacs modeline comment to make it display in Makefile mode.

Not part of the Mozilla build.

js/src/Makefile.ref

@@ -1,3 +1,4 @@
+# -*- Mode: makefile -*-
 #
 # The contents of this file are subject to the Netscape Public
 # License Version 1.1 (the "License"); you may not use this file