MINGW: config.mak.uname: drop -DNOGDI

On MinGW-W64, MsgWaitForMultipleObjects is guarded with #ifndef NOGDI.

Unfortunately, including wingdi.h brings in #define ERROR 0 which
conflicts with several Git internal enums. So, #undef ERROR.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>