fix mac compile err

src/os/src/darwin/darwinSystem.c

+/*
+ * Copyright (c) 2019 TAOS Data, Inc. <jhtao@taosdata.com>
+ *
+ * This program is free software: you can use, redistribute, and/or modify
+ * it under the terms of the GNU Affero General Public License, version 3
+ * or later ("AGPL"), as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#define _DEFAULT_SOURCE
+#include "os.h"
+#include "tglobal.h"
+#include "tulog.h"
+
+void* taosLoadDll(const char *filename) {
+  return NULL;
+}
+
+void* taosLoadSym(void* handle, char* name) {
+  return NULL;
+}
+
+void taosCloseDll(void *handle) {
+}
+
+