watch: define HOST_NAME_MAX

Those infernal arches of kfreebsd-i386 (not -amd64) don't define
HOST_NAME_MAX. This patch is a work-around for those systems with
lacking include files.
1 job for master in 2 minutes and 13 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #2285914
MakeCheck

00:02:13