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
 }

vlib/sync/sync_windows.c.v

@@ -8,6 +8,8 @@ module sync
 type MHANDLE voidptr
 
 //[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 {
 mut:
 	mx           MHANDLE    // mutex handle

vlib/sync/waitgroup.v

@@ -2,7 +2,9 @@
 // Use of this source code is governed by an MIT license
 // that can be found in the LICENSE file.
 module sync
+
 // [init_with=new_waitgroup] // TODO: implement support for init_with struct attribute, and disallow WaitGroup{} from outside the sync.new_waitgroup() function.
+[ref_only]
 pub struct WaitGroup {
 mut:
 	mu     &Mutex = &Mutex(0)