Fixed wxMessageBox with only an OK button returning wxCANCEL under MSW.

Fixed wxMessageBox with only an OK button returning wxCANCEL under MSW.

Since r67620 when wxMessageDialog::ShowModal uses a native task dialog and only has an OK button it actually uses a Cancel button, this resulted in the function's return value wrongly changing to wxID_CANCEL. Fix this by handling the special case with only an OK button and return wxID_OK instead of wxID_CANCEL (and thus wxMessageBox, which uses wxMessageDialog::ShowModal, returning wxOK instead of wxCANCEL).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67771 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775