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>