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 <[e-mail]>
Acked-by: Neil Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/sctp/checksum.h

···
  * be incorporated into the next SCTP release.
  */
 
+#ifndef __sctp_checksum_h__
+#define __sctp_checksum_h__
+
 #include <linux/types.h>
 #include <net/sctp/sctp.h>
 #include <linux/crc32c.h>
···
 {
 	return cpu_to_le32(~crc32);
 }
+
+#endif /* __sctp_checksum_h__ */