Fix build failure (missing include of <optional>)

[stepnem]

E.g.: ../src/pkgfile.hh:76:8: error: ‘optional’ in namespace ‘std’ does not name a template type 76 | std::optionalpkgfile::Result ProcessRepo(const std::filesystem::path repo, | ^~~~~~~~ ../src/pkgfile.hh:9:1: note: ‘std::optional’ is defined in header ‘’; did you forget to ‘#include ’?