netfilter: ipv6: using csum_ipv6_magic requires net/ip6_checksum.h

Fixes this build error:

net/ipv6/netfilter/nf_nat_l3proto_ipv6.c: In function 'nf_nat_ipv6_csum_recalc':
net/ipv6/netfilter/nf_nat_l3proto_ipv6.c:144:4: error: implicit declaration of function 'csum_ipv6_magic' [-Werror=implicit-function-declaration]

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>