Use string.h instead of string

Author: lucic71

rules/cstring/src.cpp

@@ -1,6 +1,6 @@
 // Copyright (c) 2022-present INESC-ID.
 // Distributed under the MIT license that can be found in the LICENSE file.
 
-#include <cstring>
+#include <string.h>
 
 const char *f6(const char *a0, int a1) { return strchr(a0, a1); }