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>