Fix leak in QDragManager::drag on mac

Change-Id: I467129f989b6e28078c9dd789cde7ff898faf1f5
Reviewed-by: Christoph Schleifenbaum <christoph.schleifenbaum@kdab.com>
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>