qthread_win.cpp: Fix warnings when waiting for adopted threads.

Task-number: QTBUG-35591

Change-Id: I63169bd8a9758a7dad33d4231d3d6c9d71c7e252
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from qtbase/d1f8a5641615eb83bb36b4ada5913531d0da24f1)
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>