RTX extern task

Hi,

i want to put my tasks into different files but i have some problems
i already searched the forum and found this thread: http://www.keil.com/forum/16857/

in my main.c

#include <rtl.h>
#include "task_led.h"
extern __task void tsk_pwr_led(void);

in my task_led.h

#ifndef _TASK_LED_H
#define _TASK_LED_H
#include <rtl.h>
__task void tsk_pwr_led(void);
#endif

in my task_led.c

__task void tsk_pwr_led(void){
  .
  .
  .
}

but during linking i get the message

.\output\reader_stm32f2.axf: Error: L6218E: Undefined symbol tsk_pwr_led (referred from main.o)

At the moment i don't see what i am doing wrong.
I don't understand the posting with #ifdef RTX_KERNEL in the other thread. do i have to define this in my source code for get this to work?

can anybody help me?