timex: introduce USEC_IN_NS constant

sys/include/timex.h

@@ -43,6 +43,11 @@ extern "C" {
  */
 #define MS_IN_USEC  (1000U)
 
+/**
+ * @brief The number of nanoseconds per microsecond
+ */
+#define USEC_IN_NS  (1000)
+
 /**
  * @brief The maximum length of the string representation of a timex timestamp
  */

sys/posix/semaphore/posix_semaphore.c

@@ -32,8 +32,6 @@
 
 #include "semaphore.h"
 
-#define USEC_IN_NS  (1000)
-
 int sem_timedwait(sem_t *sem, const struct timespec *abstime)
 {
     timex_t now, timeout = { abstime->tv_sec, abstime->tv_nsec / USEC_IN_NS };