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
Differential Revision: https://reviews.freebsd.org/D28435

(cherry picked from commit 6c34d8ce11cb73e718fc23d85b6fbfab07ee15b4)