netfilter: combine IPv4 and IPv6 nf_nat_redirect code in one module

This resolves linking problems with CONFIG_IPV6=n:

net/built-in.o: In function `redirect_tg6':
xt_REDIRECT.c:(.text+0x6d021): undefined reference to `nf_nat_redirect_ipv6'

Reported-by: Andreas Ruprecht <rupran@einserver.de>
Reported-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/ipv4/nf_nat_redirect.h

@@ -1,9 +0,0 @@
-#ifndef _NF_NAT_REDIRECT_IPV4_H_
-#define _NF_NAT_REDIRECT_IPV4_H_
-
-unsigned int
-nf_nat_redirect_ipv4(struct sk_buff *skb,
-		     const struct nf_nat_ipv4_multi_range_compat *mr,
-		     unsigned int hooknum);
-
-#endif /* _NF_NAT_REDIRECT_IPV4_H_ */

include/net/netfilter/ipv6/nf_nat_redirect.h

@@ -1,8 +0,0 @@
-#ifndef _NF_NAT_REDIRECT_IPV6_H_
-#define _NF_NAT_REDIRECT_IPV6_H_
-
-unsigned int
-nf_nat_redirect_ipv6(struct sk_buff *skb, const struct nf_nat_range *range,
-		     unsigned int hooknum);
-
-#endif /* _NF_NAT_REDIRECT_IPV6_H_ */

include/net/netfilter/nf_nat_redirect.h

@@ -0,0 +1,12 @@
+#ifndef _NF_NAT_REDIRECT_H_
+#define _NF_NAT_REDIRECT_H_
+
+unsigned int
+nf_nat_redirect_ipv4(struct sk_buff *skb,
+		     const struct nf_nat_ipv4_multi_range_compat *mr,
+		     unsigned int hooknum);
+unsigned int
+nf_nat_redirect_ipv6(struct sk_buff *skb, const struct nf_nat_range *range,
+		     unsigned int hooknum);
+
+#endif /* _NF_NAT_REDIRECT_H_ */