Fix <?xml-stylesheet?> src/href mixup to show event button icon.

calendar/lightning/content/messenger-overlay-toolbar.xul

@@ -1,5 +1,5 @@
 <?xml version="1.0"?>
-<?xml-stylesheet src="chrome://calendar/skin/calendar-toolbar.css" type="text/css"?>
+<?xml-stylesheet href="chrome://calendar/skin/calendar-toolbar.css" type="text/css"?>
 
 <overlay xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">