Fix wxMSW build in UTF-8 build.

Need an explicit cast to convert the buffer returned by wxString::t_str() to a
pointer in this case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77943 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775