net/openldap24-client: Install libldap as symlinks to libldap_r.

As of FreeBSD 12.0, OpenSSL is depending on the threading library
and it is no longer possible to build a true non-threaded version
of libldap.

The libldap_r is ABI compatible with libldap and is shipped as the
libldap on Solaris and many Linux distributions, so follow the same
approach.