add xul namespace to scrollbars agent sheet

chrome/overlay_scrollbars.as.css

@@ -4,6 +4,8 @@ See the above repository for updates as well as full license text. */
 /* This is supposed to be loaded as a agent sheet via autoconfig */
 /* It will not work otherwise and might do unexpected things */
 
+@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
+
 scrollbar {
   -moz-appearance: none!important;
   position: relative!important;