Re-add opt_mac.h include, which is required in order for MNT_MULTILABEL

to be set properly on devfs.  Otherwise, it isn't possible to set labels
on /dev nodes.

Reported by:	Sergio Rodriguez <sergiorr at yahoo.com>
MFC after:	3 days

sys/fs/devfs/devfs_vfsops.c

@@ -34,6 +34,8 @@
  * $FreeBSD$
  */
 
+#include "opt_mac.h"		/* To set MNT_MULTILABEL. */
+
 #include <sys/param.h>
 #include <sys/systm.h>
 #include <sys/kernel.h>