hook login_yubikey to the build

ok deraadt

src/etc/mtree/4.4BSD.dist

@@ -1,4 +1,4 @@
-#	$OpenBSD: 4.4BSD.dist,v 1.219 2011/09/22 04:56:09 schwarze Exp $
+#	$OpenBSD: 4.4BSD.dist,v 1.220 2012/01/26 18:45:41 dhill Exp $
 
 /set type=dir uname=root gname=wheel mode=0755
 # .
@@ -1568,6 +1568,11 @@ sysmerge
 # ./var/db/sysmerge
 ..
 
+# ./var/db/yubikey
+yubikey          uname=root gname=auth mode=0770
+# ./var/db/yubikey
+..
+
 # ./var/db
 ..