kvm: Provide kvm_vcpu_eligible_for_directed_yield() stub

Commit 7940876e ("kvm: make local
functions static") broke KVM PPC builds due to removing (rather than
moving) the stub version of kvm_vcpu_eligible_for_directed_yield().

This patch reintroduces it.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Alexander Graf <agraf@suse.de>
[Move the #ifdef inside the function. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>