Update mingw-w64 headers

[Biswapriyo Nath]

I did not mean the release page [1]. I mean the cygwin headers is older
than the current mingw-w64 git repository [2]. The latest tag is v6.0.0.
[3]

Wait, did you mean there should be a new "stable" tag or release in
mingw-w64 so that cygwin can import it?

[1]: http://mingw-w64.org/doku.php/download
[2]: https://sourceforge.net/p/mingw-w64/mingw-w64/ci/master/tree/
[3]: https://sourceforge.net/p/mingw-w64/mingw-w64/ref/master/tags/

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