x86: export check_tsc_unstable

Exporrt check_tsc_unstable function as GPL symbol. lguest is
a user of it.

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>