irq: clean up irq stat methods

David Miller suggested, related to a kstat_irqs related build breakage:

> Either linux/kernel_stat.h provides the kstat_incr_irqs_this_cpu
> interface or linux/irq.h does, not both.

So move them to kernel_stat.h.

Signed-off-by: Ingo Molnar <mingo@elte.hu>