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>