[IPV6]: Add missing XFRM select in Kconfig.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@redhat.com>

net/ipv6/Kconfig

@@ -59,6 +59,7 @@ config INET6_IPCOMP
 config IPV6_TUNNEL
 	tristate "IPv6: IPv6-in-IPv6 tunnel"
 	depends on IPV6
+	select XFRM
 	---help---
 	  Support for IPv6-in-IPv6 tunnels described in RFC 2473.