Fix building against Exiv2 main branch
The current main branch of Exiv2 has an API change that makes these fields private: https://github.com/Exiv2/exiv2/commit/c9d0cf36434c351a4dcf289d407ac70252a8ed72
The current main branch of Exiv2 has an API change that makes these fields private: https://github.com/Exiv2/exiv2/commit/c9d0cf36434c351a4dcf289d407ac70252a8ed72