ssh: Consolidate HAVE_LDNS / LIBWRAP in ssh.mk

Commit 9d63429fa163 ("ssh: move common Makefile boilerplate to a new
ssh.mk") introduced ssh.mk for common OpenSSH paths and flags, as part
of enabling FIDO/U2F. Move duplicated MK_LDNS and MK_TCP_WRAPPERS
handling there.

Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31896

(cherry picked from commit d71e7e57fc1472e3ea6d31c44e187c2819d2c71e)