Remove redundant include

Signed-off-by: Andrei Miga <andrei.miga2003@gmail.com>

src/os_threadpool.c

 // SPDX-License-Identifier: BSD-3-Clause
 
 #include <pthread.h>
-#include <semaphore.h>
 #include <stdatomic.h>
 #include <stdlib.h>
 #include <stdio.h>