Register QPlatformDialogHelper::ButtonLayout

This allows QML DialogButtonBox in Qt Quick Controls 2 to use the same
enum without having to duplicate it.

Change-Id: Ib3a89acd18f9e584a203ee44621ca2e3fc0ceaba
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/gui/kernel/qplatformdialoghelper.h

@@ -150,6 +150,7 @@ public:
         MacModelessLayout,
         AndroidLayout
     };
+    Q_ENUM(ButtonLayout)
 
     QPlatformDialogHelper();
     virtual ~QPlatformDialogHelper();