WINDOWS_EXPORT_ALL_SYMBOLS: Do not export C++ operators declared extern "C"

The pattern `extern "C" { inline bool operator==(...) {} }` appears
in at least one Windows SDK header, `winnt.h`.  Translation units that
instantiate the inline symbol produce object files with a symbol named
just `==`.  Avoid exporting such symbols because the linker will not
recognize them.

Fixes: #24999