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

drivers/clocksource/arm_global_timer.c

@@ -51,7 +51,7 @@
  * the units for all operations.
  */
 static void __iomem *gt_base;
-struct notifier_block gt_clk_rate_change_nb;
+static struct notifier_block gt_clk_rate_change_nb;
 static u32 gt_psv_new, gt_psv_bck, gt_target_rate;
 static int gt_ppi;
 static struct clock_event_device __percpu *gt_evt;