export host after setting it.

This is needed so passwd diffs show the hostname instead of
" passwd diffs:"

PR:		17651
Submitted by:	Giorgos Keramidas <keramida@ceid.upatras.gr>

usr.sbin/periodic/periodic.sh

@@ -52,6 +52,7 @@ else
 fi
 
 host=`hostname`
+export host
 echo "Subject: $host $run run output"
 
 # Execute each executable file in the directory list.  If the x bit is not