pktgen: add needed include file

Fixes this on PowerPC (at least):

net/core/pktgen.c: In function 'fill_packet_ipv6':
net/core/pktgen.c:2906:3: error: implicit declaration of function 'csum_ipv6_magic' [-Werror=implicit-function-declaration]
   udph->check = ~csum_ipv6_magic(&iph->saddr, &iph->daddr, udplen, IPPROTO_UDP, 0);
   ^

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