ftrace: fix modular build

fix:

ERROR: "start_critical_timings" [drivers/acpi/processor.ko] undefined!
ERROR: "stop_critical_timings" [drivers/acpi/processor.ko] undefined!

Signed-off-by: Ingo Molnar <[e-mail]>

kernel/trace/trace_irqsoff.c

···
 	if (preempt_trace() || irq_trace())
 		start_critical_timing(CALLER_ADDR0, CALLER_ADDR1);
 }
+EXPORT_SYMBOL_GPL(start_critical_timings);
 
 void stop_critical_timings(void)
 {
 	if (preempt_trace() || irq_trace())
 		stop_critical_timing(CALLER_ADDR0, CALLER_ADDR1);
 }
+EXPORT_SYMBOL_GPL(stop_critical_timings);
 
 #ifdef CONFIG_IRQSOFF_TRACER
 #ifdef CONFIG_PROVE_LOCKING