Update private window headerbar file name

Author: rafaelmardojai

theme/parts/headerbar-private-urlbar.css renamed to theme/parts/headerbar-private-window.css

@@ -1,6 +1,6 @@
 /* Header bar */
 @import "headerbar-urlbar.css";
-@import "headerbar-private-urlbar.css";
+@import "headerbar-private-window.css";
 @namespace xul "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
 
 /* Headerbar */