MAINTAINERS: use KVM s390x maintainers for kvm-stubs.c and kvm_s390x.h

Forgot it when factoring code out into these files. This is 100% s390x
KVM material.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170928134609.16985-2-david@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

MAINTAINERS

@@ -299,6 +299,8 @@ M: Cornelia Huck <cohuck@redhat.com>
 M: Alexander Graf <agraf@suse.de>
 S: Maintained
 F: target/s390x/kvm.c
+F: target/s390x/kvm_s390x.h
+F: target/s390x/kvm-stub.c
 F: target/s390x/ioinst.[ch]
 F: target/s390x/machine.c
 F: hw/intc/s390_flic.c