KVM: PPC: Guard doorbell exception with CONFIG_PPC_DOORBELL

Availablity of the doorbell_exception function is guarded by
CONFIG_PPC_DOORBELL. Use the same define to guard our caller
of it.
Signed-off-by: NTiejun Chen <tiejun.chen@windriver.com>
[agraf: improve patch description]
Signed-off-by: NAlexander Graf <agraf@suse.de>

arch/powerpc/kvm/booke.c

@@ -795,7 +795,7 @@ static void kvmppc_restart_interrupt(struct kvm_vcpu *vcpu,
 		kvmppc_fill_pt_regs(&regs);
 		timer_interrupt(&regs);
 		break;
-#if defined(CONFIG_PPC_FSL_BOOK3E) || defined(CONFIG_PPC_BOOK3E_64)
+#if defined(CONFIG_PPC_DOORBELL)
 	case BOOKE_INTERRUPT_DOORBELL:
 		kvmppc_fill_pt_regs(&regs);
 		doorbell_exception(&regs);