csh: Remove gethost dependency on tc.const.h

gethost is a build tool built in stage 2.3, but it had a dependency on
tc.const.h, which requires target headers (that are not installed until
stage 4.1). The build falls back to the host's headers if the target
headers don't yet exist, which may result in a build failure if the
host's headers don't match the target.

As gethost.c doesn't actually require the definitions in tc.const.h, add
a hack to skip the include of tc.const.h and remove the dependency.

PR: 283273
Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Fixes: e754e5f36195 ("Upgrade to 6.10")
Differential Revision: https://reviews.freebsd.org/D48880