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>

arch/x86/kernel/tsc_64.c

@@ -92,10 +92,12 @@ sched_clock(void) __attribute__((alias("native_sched_clock")));
 
 static int tsc_unstable;
 
-inline int check_tsc_unstable(void)
+int check_tsc_unstable(void)
 {
 	return tsc_unstable;
 }
+EXPORT_SYMBOL_GPL(check_tsc_unstable);
+
 #ifdef CONFIG_CPU_FREQ
 
 /* Frequency scaling support. Adjust the TSC based timer when the cpu frequency