ipv6: compile fix for ip6mr.c

net/ipv6/ip6mr.c: In function 'pim6_rcv':
net/ipv6/ip6mr.c:368: error: implicit declaration of function 'csum_ipv6_magic'

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>