scsi: cxlflash: Add include guards to backend.h

The new header file, backend.h, that was recently added is missing the
include guards. This commit adds the guards.

Signed-off-by: Uma Krishnan <ukrishn@linux.vnet.ibm.com>
Acked-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>