Add errno in cpp2rust compat layer

Author: lucic71

cpp2rust/compat/errno.h

@@ -0,0 +1,10 @@
+// Copyright (c) 2022-present INESC-ID.
+// Distributed under the MIT license that can be found in the LICENSE file.
+
+#include_next <errno.h>
+
+#undef errno
+
+int *cpp2rust_errno(void);
+
+#define errno (*cpp2rust_errno())