irqstat: Move declaration into asm-generic/hardirq.h

Move the declaration of the irq_cpustat per cpu variable to
asm-generic/hardirq.h and remove the now empty linux/irq_cpustat.h header.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20201113141733.737377332@linutronix.de