IPVS: include net/ip6_checksum.h for csum_ipv6_magic

Fixes this build error:

net/netfilter/ipvs/ip_vs_core.c: In function 'ip_vs_nat_icmp_v6':
net/netfilter/ipvs/ip_vs_core.c:640: error: implicit declaration of function 'csum_ipv6_magic'
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netfilter/ipvs/ip_vs_core.c

@@ -40,6 +40,7 @@
 #include <net/udp.h>
 #include <net/icmp.h>                   /* for icmp_send */
 #include <net/route.h>
+#include <net/ip6_checksum.h>
 
 #include <linux/netfilter.h>
 #include <linux/netfilter_ipv4.h>