xen: fix non-ANSI function warning in irq.c

Fix sparse warning for non-ANSI function declaration:

arch/x86/xen/irq.c:129:30: warning: non-ANSI function declaration of function 'xen_init_irq_ops'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc:	Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/irq.c

@@ -126,7 +126,7 @@ static const struct pv_irq_ops xen_irq_ops __initdata = {
 #endif
 };
 
-void __init xen_init_irq_ops()
+void __init xen_init_irq_ops(void)
 {
 	pv_irq_ops = xen_irq_ops;
 	x86_init.irqs.intr_init = xen_init_IRQ;