powerpc: Add missing prototype for arch_irq_work_raise()

In commit 4f8b50bb ("irq_work, ppc: Fix up arch hooks") a new
function arch_irq_work_raise() was added without a prototype in header
irq_work.h.

Fix the following warning (treated as error in W=1):
  arch/powerpc/kernel/time.c:523:6: error: no previous prototype for ‘arch_irq_work_raise’

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>