KVM: put back #ifndef CONFIG_S390 around kvm_vcpu_kick

The #ifndef was removed in 75aaafb7,
but it was also protecting smp_send_reschedule() in kvm_vcpu_kick().

Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>