Bug 1573026 - Include missing header in XULPersist for old XUL store. r=smaug

Beta builds use the old XUL store which was missing a header after the
removal of XULDocument.

Differential Revision: https://phabricator.services.mozilla.com/D41608

--HG--
extra : moz-landing-system : lando

dom/xul/XULPersist.cpp

@@ -10,6 +10,7 @@
 #  include "mozilla/XULStore.h"
 #else
 #  include "nsIXULStore.h"
+#  include "nsIStringEnumerator.h"
 #endif
 
 #include "nsIXULWindow.h"