userboot is missing vdisk_dev

Add vdisk device support in userboot configuration.

MFC after:	1 week

stand/userboot/userboot/conf.c

@@ -42,6 +42,8 @@ __FBSDID("$FreeBSD$");
 #include "libzfs.h"
 #endif
 
+extern struct devsw vdisk_dev;
+
 /*
  * We could use linker sets for some or all of these, but
  * then we would have to control what ended up linked into
@@ -55,6 +57,7 @@ __FBSDID("$FreeBSD$");
 struct devsw *devsw[] = {
 	&host_dev,
 	&userboot_disk,
+	&vdisk_dev,
 #if defined(USERBOOT_ZFS_SUPPORT)
 	&zfs_dev,
 #endif