KVM: Use CONFIG_PREEMPT_NOTIFIERS around struct preempt_notifier

This allows kvm_host.h to be #included even when struct preempt_notifier is
undefined. This is needed to build ppc asm-offsets.h.

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 928b0d59..b90ca36 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -67,7 +67,9 @@
 
 struct kvm_vcpu {
 	struct kvm *kvm;
+#ifdef CONFIG_PREEMPT_NOTIFIERS
 	struct preempt_notifier preempt_notifier;
+#endif
 	int vcpu_id;
 	struct mutex mutex;
 	int   cpu;