s390/MAINTAINERS: add gmap.c to kvm maintainers

gmap.c deals mostly with KVM-related memory management, so a lot
of changes to this file will come via the KVM tree. Reflect this
in MAINTAINERS. Please note that there are intricate ties to
arch/s390/mm/pgtable.c. If changes are needed in both files,
this will continue to be submitted via the s390 tree (or a
topic branch if necessary).

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>