openvswitch: Include ip6_fib.h.

kbuild test robot reports that certain configurations will not
automatically pick up on the "struct rt6_info" definition, so explicitly
include the header for this structure.

Fixes: 7f8a436e "openvswitch: Add conntrack action"
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>