Add benchmarks for json string serialize/deserialize (#12258)

Co-authored-by: sfan5 <sfan5@live.de>

src/benchmark/benchmark.h

@@ -0,0 +1,26 @@
+/*
+Minetest
+Copyright (C) 2022 Minetest Authors
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 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 Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser 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.
+*/
+
+#pragma once
+
+#include "config.h"
+
+#if BUILD_BENCHMARKS
+extern int run_benchmarks();
+#endif