[NETFILTER]: Fix build with CONFIG_NETFILTER=y/m on IA64

Can't build with CONFIG_NETFILTER=y/m on IA64, there's a missing
#include in net/ipv6/netfilter.c

net/ipv6/netfilter.c: In function `nf_ip6_checksum':
net/ipv6/netfilter.c:92: warning: implicit declaration of function
`csum_ipv6_magic'

Signed-off-by: Brian Haley <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv6/netfilter.c

···
 #include <net/ipv6.h>
 #include <net/ip6_route.h>
 #include <net/xfrm.h>
+#include <net/ip6_checksum.h>
 
 int ip6_route_me_harder(struct sk_buff *skb)
 {