fix: added include required for macOS 14

server/src/main.cpp

@@ -3,7 +3,7 @@
  *
  * This file is part of the traintastic source code.
  *
- * Copyright (C) 2019-2023 Reinder Feenstra
+ * Copyright (C) 2019-2024 Reinder Feenstra
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
@@ -21,6 +21,7 @@
  */
 
 #include <iostream>
+#include <functional>
 #include "options.hpp"
 #include "traintastic/traintastic.hpp"
 #include "log/log.hpp"