lib: os: coding guidelines: add explicit cast to void

- added explicit cast to void when returned value is expectedly ignored

Signed-off-by: frei tycho <tfrei@baumer.com>
diff --git a/lib/os/sem.c b/lib/os/sem.c
index b01ead5..b7676c9 100644
--- a/lib/os/sem.c
+++ b/lib/os/sem.c
@@ -53,7 +53,7 @@
 		return -EINVAL;
 	}
 
-	atomic_set(&sem->futex.val, initial_count);
+	(void)atomic_set(&sem->futex.val, initial_count);
 	sem->limit = limit;
 
 	return 0;