sys_stat: Fix support for mingw64 and MSVC. * lib/sys_stat.in.h (stat) [AIX]: Don't redefine 'stat' if the system header files already do it. (stat) [mingw, msvc]: Redefine the symbol to which stat is defined, not stat itself. Reported by Marc-André Lureau <marcandre.lureau@redhat.com>.