gethostname(3): Correct the reference of the undefined value HOST_NAME_MAX

PR: 143472
Submitted by: bcran@
Reported by: Jeremy Huddleston <jeremyhu at apple dot com>
Reviewed by: bcr (mentor)
Approved by: bcr (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25647