net: sctp: prevent checksum.h from double inclusion

The header file checksum.h is missing proper defines that prevents
it from double inclusion.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>