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>

MAINTAINERS

@@ -7170,6 +7170,7 @@ S:	Supported
 F:	Documentation/s390/kvm.txt
 F:	arch/s390/include/asm/kvm*
 F:	arch/s390/kvm/
+F:	arch/s390/mm/gmap.c
 
 KERNEL VIRTUAL MACHINE (KVM) FOR ARM
 M:	Christoffer Dall <christoffer.dall@linaro.org>