x86_64: Select clocksource watchdog in Kconfig

TSC must be verified by a continous and reliable clocksource to
allow high resolution timers and or dynamic ticks.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>