OpenBSD build fails because of lack of EVP_add_digest()

[demonfoo]

It appears that with OpenBSD 7.5 (which uses LibreSSL 3.9.0), Net::SSLeay fails to build. After a bit of research, I've discovered that that's because LibreSSL 3.9.0 dropped support for EVP_add_digest() (and EVP_add_cipher()).

[demonfoo]

#485

I created the above PR to conditionalize the inclusion of the function such that it won't be wrapped with LibreSSL 3.9.0; running a build and make test in an OpenBSD VM resulted in no test failures.