Move wx/msw/gccpriv.h inclusion back to wx/platform.h from wx/compiler.h.

Move wx/msw/gccpriv.h inclusion back to wx/platform.h from wx/compiler.h.

Do keep compiler-specific wxCHECK_MINGW32_VERSION() in wx/compiler.h as it's
needed by wx/cpp.h which includes wx/compiler.h only. But put the rest of the
stuff in gccpriv.h in its old place as including it before wx/setup.h didn't
work correctly and was unnecessary anyhow.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74506 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775