MAINTAINERS: add virtio-ccw.h to virtio/s390 section

The file arch/s390/include/uapi/asm/virtio-ccw.h belongs to the
s390 virtio drivers as well.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>