fix: extern cause clang link failure

[reitowo]

Use clang-cl 17.0.6 / 18.1.8 failed to compile the code using CMake, with link error:

lld-link: error: undefined symbol: void __cdecl google::protobuf::internal::memswap<12>(char *__restrict, char *__restrict)
>>> referenced by xxxxx\cmake-build-debug-win-x86-md-17\stackwalker.pb.cc:1179
>>>               xxx.lib(stackwalker.pb.cc.obj):(private: void __thiscall xxx::protobuf::OnDbgHelpErrRequest::InternalSwap(class xxx::protobuf::OnDbgHelpErrRequest *))
>>> referenced by xxxxx\cmake-build-debug-win-x86-md-17\vcpkg_installed\x86-windows-static-md\include\google\protobuf\repeated_ptr_field.h:330
>>>               xxx.lib(stackwalker.pb.cc.obj):(public: void __thiscall google::protobuf::internal::RepeatedPtrFieldBase::InternalSwap(class google::protobuf::internal::RepeatedPtrFieldBase *__restrict))
ninja: build stopped: subcommand failed.

It seems LLVM failed to keep the symbol as extern, or it simply a bug to let the code compile using MSVC. Either use extern "C" or remove extern solves the problem. I use extern "C" to keep the original behavior.