bug#48303: 28.0.50; Build of master fails on MSYS2 MINGW64 environment

[Eli Zaretskii]

> Date: Sun, 09 May 2021 07:35:09 +0900 (JST)
> From: Yasuhiro Kimura <yasu@utahime.org>
> 
> In file included from w32image.c:32:
> w32common.h:45:3: error: expected identifier before numeric constant
>    45 |   OS_NT
>       |   ^~~~~
> make[1]: *** [Makefile:385: w32image.o] Error 1
> make[1]: Leaving directory '/c/Users/yasu/Work/Emacs/master/emacs/src'
> make: *** [Makefile:425: src] Error 2

That's because w32 API headers now invade on our enumeration.  Fixed
on the emacs-27 branch, soon to be merged to master.

Thanks.