Define XML_STATIC for Expat build to fix linking of Windows DLLs.

The new Expat assumes it's built as a DLL by default, define XML_STATIC to
indicate that this is not the case as we always build it as a static library.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74444 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775