cnic: Need to include net/ip6_checksum.h

drivers/net/cnic.c: In function 'cnic_init_storm_conn_bufs':
drivers/net/cnic.c:1757: error: implicit declaration of function 'csum_ipv6_magic'

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