openssh: disable libwrap (TCP wrappers) at configure time

We define LIBWRAP at build time in secure/usr.sbin/sshd/Makefile if
WITH_TCPWRAPPERS is in effect, so it should not be set in config.h.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 36cd1e5e8c216e0f67832d59d629f2910a02d4fc)