ktls(4): Mention WITH_OPENSSL_KTLS in the ktls(4) manual page
Since we ship a ktls(4) enabled OpenSSL version, mention
the src.conf(5) option WITH_OPENSSL_KTLS in the manual page.
Reviewed by: jhb
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D28435