tests: do not build ktls_test if WITHOUT_OPENSSL

ktls_test requires libcrypto to build, and fails if it is not available
(which is the case when building WITHOUT_OPENSSL).

Reported by: Michael Dexter, Build Option Survey
Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32895

(cherry picked from commit e818178e3a597922ce77bee874e441a1b7df0099)