d-pointerize QWinOverlappedIoNotifier

Preparation for removing the qt_windows.h include from
qwinoverlappedionotifier_p.h.

Change-Id: I27ab3891962327ab5db75fbfcc3cf57c50a0d1c4
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>