Mark qtmultimedia as an optional dependency for qtwinextras

The QtWinExtras Music Player example uses QtMultimedia.

Change-Id: Ib720710a8563d6bfa39aec8b82e9a87517b7e015
Reviewed-by: Mandeep Sandhu <mandeepsandhu.chd@gmail.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Updated submodules.

Change-Id: Ie937791dcbe07d65728ab2cea1d590ab1eec8922
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>