Unfortunately we can't keep it in wx/compiler.h which is included too early,
before UNICODE macro is defined, and we can't define it before including
wx/setup.h which itself must be included after wx/compiler.h.
So move it to wx/msw/gccpriv.h which is not a great solution, as the best
would be to get rid of this gcc-specific file completely, but at least should
make everything build again and ensure that _mingw.h is included after UNICODE
definition.
Closes#15805.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75555 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
35ae03b3b2