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>

arch/powerpc/include/asm/irq_work.h

@@ -6,5 +6,6 @@ static inline bool arch_irq_work_has_interrupt(void)
 {
 	return true;
 }
+extern void arch_irq_work_raise(void);
 
 #endif /* _ASM_POWERPC_IRQ_WORK_H */