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>
diff --git a/net/netfilter/Makefile b/net/netfilter/Makefile
index f3eb468..89f73a9 100644
--- a/net/netfilter/Makefile
+++ b/net/netfilter/Makefile
@@ -51,6 +51,7 @@
 obj-$(CONFIG_NF_LOG_COMMON) += nf_log_common.o
 
 obj-$(CONFIG_NF_NAT) += nf_nat.o
+obj-$(CONFIG_NF_NAT_REDIRECT) += nf_nat_redirect.o
 
 # NAT protocols (nf_nat)
 obj-$(CONFIG_NF_NAT_PROTO_DCCP) += nf_nat_proto_dccp.o