header: add NETIF_F_SCTP_CRC

NETIF_F_SCTP_CSUM was renamed to NETIF_F_SCTP_CRC in commit 53692b1d


"sctp: Rename NETIF_F_SCTP_CSUM to NETIF_F_SCTP_CRC" make new code
still work with old kernel.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>