Avoid duplicate signed char generic association with MSVC

[crazywhalecc]

What does this PR do?

MSVC rejects the nghttp3_max and nghttp3_min _Generic association lists because both signed char and char are present and are treated as compatible types, which causes C7700 errors when building nghttp3 with Visual Studio 2022.

This change omits the signed char generic association only for MSVC, while preserving the existing behavior for other compilers. The actual nghttp3 call sites use integer, size_t, and uint64_t values, so this avoids the MSVC compile failure without changing the non-MSVC path.

[gemini-code-assist]

Code Review

This pull request introduces conditional compilation macros to omit the signed char association from _Generic selection lists in nghttp3_max and nghttp3_min when compiling with MSVC. The review feedback correctly identifies that newer versions of MSVC (VS 2022 17.14+) and Clang conformantly treat signed char as a distinct type, meaning unconditional omission will lead to compilation failures. It is recommended to update the preprocessor checks to only omit the association on older MSVC versions when not compiling with Clang.

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