ipv6: Fix build error with udp_offload

Add ip6_checksum.h include.  This should resolve the following issue
that shows up on power:

net/ipv6/udp_offload.c: In function 'udp6_ufo_send_check':
net/ipv6/udp_offload.c:29:2: error: implicit declaration of function
'csum_ipv6_magic' [-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>