m68knommu: Move forward declaration of do_IRQ() from machdep.h to irq.h

It is not machine-specific, but common irq infrastructure.
Also add the missing asmlinkage, to match its definition.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>