clocksource/drivers/arm_global_timer: Make symbol 'gt_clk_rate_change_nb' static

The sparse tool complains as follows:

drivers/clocksource/arm_global_timer.c:54:23: warning:
 symbol 'gt_clk_rate_change_nb' was not declared. Should it be static?

This symbol is not used outside of arm_global_timer.c, so mark it static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1623490046-37972-1-git-send-email-zou_wei@huawei.com