Give QWindowsNativeFileDialogEventHandler a virtual destructor

GCC complains about this class having virtuals and no virtual destructor.

C:/Qt/qt5/qtbase/src/plugins/platforms/windows/qwindowsdialoghelpers.cpp:750:20: warning: deleting object of polymorphic class type 'QWindowsNativeFileDialogEventHandler' which has non-virtual destructor might cause undefined behaviour [-Wdelete-non-virtual-dtor]

Change-Id: I7494fce4bb71ed65954226259fa65fdc507ca403
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>