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>