databases/mysql80-client: Fix output of mysql_config for ldflags
Make mysql_config not show non-existing libraries as output of --lib.
(imported_crypto and imported_openssl which seem to be for internal use)

This issue made build-failures on downstream codes that rely on
correct output to use for their LDFLAGS (like mail/dovecot).

Reported by: ler
Sponsored by: The FreeBSD Foundation