Fix wxCHECK_MINGW32_VERSION() definition.

Include _mingw.h before testing for __MINGW32_MAJOR_VERSION as it's only
defined in that header.

Closes #15696.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75304 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775