MAINTAINERS: update file path for S390 VFIO AP DRIVER

Changed the MAINTAINERS file to include the new
drivers/s390/crypto/vfio_ap_debug.h file path.

Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Acked-by: Matthew Rosato <mjrosato@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>