x86/tsc: Make art_related_clocksource static

The symbol is not used outside of the file, so mark it static.

Fixes the following warning:

arch/x86/kernel/tsc.c:53:20: warning:
	symbol 'art_related_clocksource' was not declared. Should it be static?

Signed-off-by: Chen Lifu <chenlifu@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20220823021821.3052159-1-chenlifu@huawei.com