m68knommu: add prototype for ack_bad_irq

Create prototype for ack_bad_irq() for m68knommu.
Compilation of kernel/irq/handle.c fails without it.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>