Create a public header file for OID values

This will be a subset of the former `<mbedtls/oid.h>`, with only macro
definitions, no function declarations.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

include/mbedtls/oid.h

@@ -0,0 +1,16 @@
+/**
+ * \file oid.h
+ *
+ * \brief Object Identifier (OID) values
+ */
+/*
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+#ifndef MBEDTLS_OID_H
+#define MBEDTLS_OID_H
+
+#include "mbedtls/build_info.h"
+
+
+#endif /* oid.h */