mail/libdomainkeys: Correct OPENSSL_VERSION_NUMBER

For the sake posterity use OpenSSL 1.1.0-pre1's version which is
when EVP_MD_CTX was made opaque [1]. This should have no practical
impact on the build with any of the OpenSSL versions we support.

[1] https://github.com/openssl/openssl/commit/7638370ca6cb

Reported by: danfe, jbeich