config: use __declspec(dllexport) on all Windows (MinGW/Clang), not just MSVC

[jcelerier]

Problem

LIBREMIDI_EXPORT resolved to __attribute__((visibility("default"))) on the GCC/Clang path:

#if defined(LIBREMIDI_EXPORTS) || defined(LIBREMIDI_MODULE_BUILD)
  #if defined(_MSC_VER)
    #define LIBREMIDI_EXPORT __declspec(dllexport)
  #elif defined(__GNUC__) || defined(__clang__)
    #define LIBREMIDI_EXPORT __attribute__((visibility("default")))
  #endif

But visibility is an ELF concept. On Windows PE/COFF (MinGW-GCC and Clang targeting *-w64-windows-gnu) it is silently inert — it produces no PE export. So on those toolchains:

• a shared libremidi (.dll) built with MinGW/Clang exports none of its API, and
• a static libremidi re-exported from an executable (e.g. a host that JIT-compiles plugins referencing libremidi::reader::parse) exports nothing either.

Only the MSVC branch got __declspec(dllexport).

Fix

Key the dllexport branch on _WIN32 so it covers all Windows compilers (MSVC and MinGW/Clang). __declspec(dllexport) is supported by GCC/Clang on mingw and is the correct PE mechanism.

Verification

On x86_64-w64-windows-gnu, compiling:

__attribute__((visibility("default"))) int vis_func() { return 1; }
__declspec(dllexport)                   int dll_func() { return 2; }

llvm-objdump -s --section=.drectve shows a -export: directive only for dll_func — vis_func produces none. Confirmed against the real symptom: a MinGW/Clang-built static libremidi shows 0 -export: directives; after this change its public API (reader, observer, available_apis, …) exports correctly.

This should be validated by the existing MSYS/MinGW CI matrix.