WinRT: Implement QFSFileEnginePrivate::unmap()

This should have gone with ad63c9a5, but here it is now. No changes
apart from letting the existing implementation be used.

This will also need to be fixed on Windows Phone.

Change-Id: I8054639fe8e3178544078a5673c168a43a883da2
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>