add missing parameter

addresses: RIOT-OS#1041 (diff)

Author: LudwigKnuepfer

core/include/hwtimer.h

@@ -138,7 +138,9 @@ int hwtimer_active(void);
 void hwtimer_cpu_init(void (*handler)(int), uint32_t fcpu);
 
 /**
- * @brief   initialize hwtimer module data structures and hardware
+ * @brief       initialize hwtimer module data structures and hardware
+ *
+ * @param[in]   fcpu        cpu frequency
  */
 void hwtimer_init_comp(uint32_t fcpu);