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>

drivers/xen/events.c

@@ -395,9 +395,9 @@ static void unmask_evtchn(int port)
 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