This patch allows the the user to build xfrm4_tunnel/xfrm6_tunnel as
modules.

This makes sense because they're only used by IPComp/IPIP/IP6Tunnel
which are modules themselves.  It also means that distros can cut
down on there core kernel size when compiling with IPsec support.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@redhat.com>