arch/mn10300/kernel/irq.c: fix build

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=25702



Reported-by: Martin Ettl <ettl.martin@gmx.de>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>