powerpc: Add missing prototype for init_IRQ()

A function init_IRQ() was added without a prototype declared in header
irq.h. Fix the following warning (treated as error in W=1):

  arch/powerpc/kernel/irq.c:662:13: error: no previous prototype for ‘init_IRQ’
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/irq.h

@@ -66,6 +66,7 @@ extern void irq_ctx_init(void);
 extern void call_do_softirq(struct thread_info *tp);
 extern void call_do_irq(struct pt_regs *regs, struct thread_info *tp);
 extern void do_IRQ(struct pt_regs *regs);
+extern void __init init_IRQ(void);
 extern void __do_irq(struct pt_regs *regs);
 
 int irq_choose_cpu(const struct cpumask *mask);