sync: unpublish LocksHeld() which is used only in sync.cpp

src/sync.cpp

@@ -246,7 +246,7 @@ void LeaveCritical()
     pop_lock();
 }
 
-std::string LocksHeld()
+static std::string LocksHeld()
 {
     LockData& lockdata = GetLockData();
     std::lock_guard<std::mutex> lock(lockdata.dd_mutex);