db_mysql module in kamailio doesn't link against libssl itself. So likely that is a dependency enforced by the libmysqlclient library. You have to check what that library does for resolving dependencies.
Also, normally, it is `pkg-config ... libssl`, not `pkg-config ... openssl`, used to find the paths for libssl.
Moreover, I can compile db_mysql just fine in my macos 10.14, with mysql and libssl installed via macports. If you installed those from sources directly, then I guess you have to sort out dependencies manyally.
Not being an issue related directly to kamailio code or files, I suggest to write to sr-users mailing list and get assistance from the community of user, from kamailio developer point of view, I don't think this is something we can fix for your case.