sparseirq: export nr_irqs on m68k/sparc/s390

Stephen Rothwell reported such build failures on m68k/sparc/s390:

> ERROR: "nr_irqs" [drivers/net/hamradio/baycom_ser_fdx.ko] undefined!
> ERROR: "nr_irqs" [drivers/net/3c59x.ko] undefined!

export nr_irqs on these architectures too.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>