lib: silence LibreSSL collision warning on non-MSVC Windows

[vszakats]

LibreSSL headers emit this warning because we included wincrypt.h
before them. We have to include wincrypt.h before OpenSSL headers
to avoid symbol collisions when using other forks. LibreSSL 3.8.2+
offers a macro to silence its warnings to avoid this issue. This patch
sets it.

This allows to stop setting this macro in curl-for-win builds.

Warnings seen with MinGW with cmake non-unity (also unity batch=30):

[156/219] Building C object lib/CMakeFiles/libcurl_object.dir/vtls/openssl.c.obj
In file included from lib/vtls/openssl.h:35,
                 from lib/vtls/openssl.c:53:
dep/libressl-win-x64/include/openssl/ossl_typ.h:90:2: warning: #warning overriding WinCrypt defines [-Wcpp]
   90 | #warning overriding WinCrypt defines
      |  ^~~~~~~
In file included from dep/libressl-win-x64/include/openssl/pem.h:71,
                 from dep/libressl-win-x64/include/openssl/ssl.h:151,
                 from lib/vtls/openssl.h:36:
dep/libressl-win-x64/include/openssl/x509.h:108:2: warning: #warning overriding WinCrypt defines [-Wcpp]
  108 | #warning overriding WinCrypt defines
      |  ^~~~~~~
In file included from dep/libressl-win-x64/include/openssl/x509.h:319:
dep/libressl-win-x64/include/openssl/pkcs7.h:77:2: warning: #warning overriding WinCrypt defines [-Wcpp]
   77 | #warning overriding WinCrypt defines
      |  ^~~~~~~

Ref: libressl/portable#910
Ref: libressl/portable#924
Ref: libressl/portable@e7fe6ca
Ref: curl/curl-for-win@760ccfc

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w/o whitespace: https://github.com/curl/curl/pull/16273/files?w=1