powerpc: Add missing prototype for arch_irq_work_raise()

powerpc: Add missing prototype for arch_irq_work_raise()

[ Upstream commit f5246862f82f1e16bbf84cda4cddf287672b30fe ]

In commit 4f8b50bbbe63 ("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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>