sync: don't force Mutex and Semaphore to be reference (#8331)

vlib/sync/waitgroup.v

@@ -14,7 +14,7 @@ module sync
 struct WaitGroup {
 mut:
 	task_count       int // current task count
-	task_count_mutex &Mutex = &Mutex(0) // This mutex protects the task_count count in add()
+	task_count_mutex &Mutex // This mutex protects the task_count count in add()
 	wait_blocker     &Waiter = &Waiter(0) // This blocks the wait() until released by add()
 }