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 <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/m68k/kernel/ints.c

···
 #endif
 
 int nr_irqs = NR_IRQS;
+EXPORT_SYMBOL(nr_irqs);
 
 extern u32 auto_irqhandler_fixup[];
 extern u32 user_irqhandler_fixup[];

arch/s390/kernel/irq.c

···
 #include <linux/profile.h>
 
 int nr_irqs = NR_IRQS;
+EXPORT_SYMBOL(nr_irqs);
 
 /*
  * show_interrupts is needed by /proc/interrupts.

arch/sparc/kernel/irq.c

···
 #endif /* SMP */
 
 int nr_irqs = NR_IRQS;
+EXPORT_SYMBOL(nr_irqs);
 
 unsigned long __raw_local_irq_save(void)
 {