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 <mingo@elte.hu>

+2
+2
kernel/trace/trace_irqsoff.c
··· 253 253 if (preempt_trace() || irq_trace()) 254 254 start_critical_timing(CALLER_ADDR0, CALLER_ADDR1); 255 255 } 256 + EXPORT_SYMBOL_GPL(start_critical_timings); 256 257 257 258 void stop_critical_timings(void) 258 259 { 259 260 if (preempt_trace() || irq_trace()) 260 261 stop_critical_timing(CALLER_ADDR0, CALLER_ADDR1); 261 262 } 263 + EXPORT_SYMBOL_GPL(stop_critical_timings); 262 264 263 265 #ifdef CONFIG_IRQSOFF_TRACER 264 266 #ifdef CONFIG_PROVE_LOCKING