dracut: Install commands required for vdev_id

The vdev_id script requires awk, grep, and head.  Use dracut_install to
ensure that these commands are available in the initrd environment.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Karsten Kretschmer <kkretschmer@gmail.com>
Closes #6443
Closes #6452

contrib/dracut/90zfs/module-setup.sh.in

@@ -52,6 +52,8 @@ install() {
 	fi
 	dracut_install @sbindir@/mount.zfs
 	dracut_install @udevdir@/vdev_id
+	dracut_install awk
+	dracut_install head
 	dracut_install @udevdir@/zvol_id
 	inst_hook cmdline 95 "${moddir}/parse-zfs.sh"
 	if [ -n "$systemdutildir" ] ; then