Including wx/msw/missing.h to define INVALID_FILE_ATTRIBUTES for MSVC6.

This header is also needed in filename.cpp (see r73317 for a previous commit
fixing the same problem in filefn.cpp).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73344 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775