build: options: mention ports in the WITH_OPENLDAP description

There's a third party dependency on this option; currently,
net/openldap24-{,sasl-}client. At least mention that an openldap from ports
is needed for this option.

PR: 252866

(cherry picked from commit 7587d9823a8257b9a2d5b2e58c707026061058c6)