add copyright notice as per GPL (#7)

src/ap_server.cpp

@@ -1,4 +1,23 @@
-#include <ctime>
+/**
+ * apsdk - API for an open-source AirPlay  server
+ * Copyright (C) 2018-2023 Sheen Tian
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 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.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301  USA
+ */
+
+#include <ctime>
 #include <memory>
 
 #include <ap_config.h>