xen: Fix compile warning when CONFIG_SMP is not defined.

.. which is quite benign.

drivers/xen/events.c:398: warning: unused variable ‘desc’

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
diff --git a/drivers/xen/events.c b/drivers/xen/events.c
index 553da68..30df85d 100644
--- a/drivers/xen/events.c
+++ b/drivers/xen/events.c
@@ -395,9 +395,9 @@
 static void xen_irq_init(unsigned irq)
 {
 	struct irq_info *info;
+#ifdef CONFIG_SMP
 	struct irq_desc *desc = irq_to_desc(irq);
 
-#ifdef CONFIG_SMP
 	/* By default all event channels notify CPU#0. */
 	cpumask_copy(desc->irq_data.affinity, cpumask_of(0));
 #endif