Including missing header for wxUniv/MSW build.

Compilation fix after the changes of r76657, see #16265.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76663 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Add cast to wxWindow to fix wxUniv/MSW compilation.

wxWindowMSW is different from (base class of) wxWindow in wxUniv and needs to
be converted to it explicitly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76664 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775