sync: mark sync.WaitGroup and sync.Mutex with [ref_only]

vlib/sync/sync_nix.c.v

@@ -4,14 +4,12 @@
 module sync
 
 #include <pthread.h>
-fn C.pthread_mutex_init()
+fn C.pthread_mutex_init(voidptr, voidptr) int
+fn C.pthread_mutex_lock(voidptr) int
+fn C.pthread_mutex_unlock(voidptr) int
 
-
-fn C.pthread_mutex_lock()
-
-
-fn C.pthread_mutex_unlock()
 // [init_with=new_mutex] // TODO: implement support for this struct attribute, and disallow Mutex{} from outside the sync.new_mutex() function.
+[ref_only]
 pub struct Mutex {
 	mutex C.pthread_mutex_t
 }