[IPSEC]: make sit use the xfrm4_tunnel_register

This patch makes sit use xfrm4_tunnel_register instead of
inet_add_protocol. It solves conflict of sit device with
inter address family IPsec tunnel.

Signed-off-by: Kazunori MIYAZAWA <miyazawa@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/Kconfig b/net/ipv6/Kconfig
index deb4101..79682ef 100644
--- a/net/ipv6/Kconfig
+++ b/net/ipv6/Kconfig
@@ -156,6 +156,7 @@
 config IPV6_SIT
 	tristate "IPv6: IPv6-in-IPv4 tunnel (SIT driver)"
 	depends on IPV6
+	select INET_TUNNEL
 	default y
 	---help---
 	  Tunneling means encapsulating data of one protocol type within