arm64: irqstat: Get rid of duplicated declaration

irq_cpustat_t is exactly the same as the asm-generic one. Define
ack_bad_irq so the generic header does not emit the generic version of it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201113141733.392015387@linutronix.de