x86/jailhouse: Set X86_FEATURE_TSC_KNOWN_FREQ

Otherwise, Linux will not recognize precalibrated_tsc_khz and disable
the tsc as clocksource.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jailhouse <jailhouse-dev@googlegroups.com>
Link: https://lkml.kernel.org/r/975fbfc9-2a64-cc56-40d5-164992ec3916@siemens.com