src.opts.mk: set MK_ZFS conditional on MK_OPENSSL

libzfs has a dependency on libcrypto. This causes a buildworld link
failure when WITHOUT_OPENSSL/WITHOUT_CRYPT is set.

This dependency was added implicitly by the switch to OpenZFS, and
explicitly in 40d0fd287510 and cd568e2b1b67.

PR: 252841
Reviewed by: kevans, freqlabs
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D28431