cryptodeflate: Drop z_stream zbuf.state->dummy from SDT probe.

For older versions of zlib, dummy was a workaround for compilers that do not
handle opaque type definition well; on FreeBSD, it's representing a value
that is not really useful for monitoring purposes, and the field would be gone
in newer zlib versions.

PR: 229763
Submitted by: Yoshihiro Ota <ota at j.email.ne.jp>
Differential Revision: https://reviews.freebsd.org/D20222