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>

net/core/pktgen.c

@@ -161,6 +161,7 @@
 #include <net/checksum.h>
 #include <net/ipv6.h>
 #include <net/udp.h>
+#include <net/ip6_checksum.h>
 #include <net/addrconf.h>
 #ifdef CONFIG_XFRM
 #include <net/xfrm.h>