Add timed wait semaphore

src/platforms/px4_posix.h

@@ -62,6 +62,7 @@ typedef struct {
 
 __EXPORT int		px4_sem_init(px4_sem_t *s, int pshared, unsigned value);
 __EXPORT int		px4_sem_wait(px4_sem_t *s);
+__EXPORT int		px4_sem_timedwait(px4_sem_t * sem, const struct timespec * abstime);
 __EXPORT int		px4_sem_post(px4_sem_t *s);
 __EXPORT int		px4_sem_getvalue(px4_sem_t *s, int *sval);
 __EXPORT int		px4_sem_destroy(px4_sem_t *s);
@@ -76,6 +77,7 @@ typedef sem_t px4_sem_t;
 
 #define px4_sem_init	 sem_init
 #define px4_sem_wait	 sem_wait
+#define px4_sem_timedwait	 sem_timedwait
 #define px4_sem_post	 sem_post
 #define px4_sem_getvalue sem_getvalue
 #define px4_sem_destroy	 sem_destroy