s390x/kvm: Fix compile error

commit a2b257d6212a "memory: expose alignment used for allocating RAM
as MemoryRegion API" triggered a compile error on KVM/s390x.

Fix the prototype and the implementation of legacy_s390_alloc.

Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
