clocksource/drivers/mips-gic-timer: Make gic_compare_irqaction static

Fix sparse warning:

drivers/clocksource/mips-gic-timer.c:70:18: warning:
 symbol 'gic_compare_irqaction' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: <daniel.lezcano@linaro.org>
Link: https://lkml.kernel.org/r/20190322144359.19516-1-yuehaibing@huawei.com