Move command log functions to their own header, fix missing include

See: #586

src/command_log.h

@@ -0,0 +1,18 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD 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, version 2.
+ * OpenTTD 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 OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file command_log.h Functions related to command logging. */
+
+#ifndef COMMAND_LOG_H
+#define COMMAND_LOG_H
+
+#include <functional>
+
+void ClearCommandLog();
+char *DumpCommandLog(char *buffer, const char *last, std::function<char *(char *)> flush);
+
+#endif /* COMMAND_LOG_H */