netfilter: nfnetlink_queue: fix compilation with CONFIG_NF_NAT=m and CONFIG_NF_CT_NETLINK=y

  LD      init/built-in.o
net/built-in.o:(.data+0x4408): undefined reference to `nf_nat_tcp_seq_adjust'
make: *** [vmlinux] Error 1

This patch adds a new pointer hook (nfq_ct_nat_hook) similar to other existing
in Netfilter to solve our complicated configuration dependencies.

Reported-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/include/linux/netfilter.h b/include/linux/netfilter.h
index 38b96a5..c613cf0 100644
--- a/include/linux/netfilter.h
+++ b/include/linux/netfilter.h
@@ -401,10 +401,14 @@
 	size_t (*build_size)(const struct nf_conn *ct);
 	int (*build)(struct sk_buff *skb, struct nf_conn *ct);
 	int (*parse)(const struct nlattr *attr, struct nf_conn *ct);
+};
+extern struct nfq_ct_hook __rcu *nfq_ct_hook;
+
+struct nfq_ct_nat_hook {
 	void (*seq_adjust)(struct sk_buff *skb, struct nf_conn *ct,
 			   u32 ctinfo, int off);
 };
-extern struct nfq_ct_hook __rcu *nfq_ct_hook;
+extern struct nfq_ct_nat_hook __rcu *nfq_ct_nat_hook;
 #else
 static inline void nf_ct_attach(struct sk_buff *new, struct sk_buff *skb) {}
 #endif