irqchip/apple-aic: Make symbol 'use_fast_ipi' static

The sparse tool complains as follows:

drivers/irqchip/irq-apple-aic.c:231:1: warning:
 symbol 'use_fast_ipi' was not declared. Should it be static?

This symbol is not used outside of irq-apple-aic.c, so marks it static.

Fixes: 2cf68211 ("irqchip/apple-aic: Add Fast IPI support")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220618072824.562350-1-weiyongjun1@huawei.com