Add certificate policy oid x509 extension

Add the `MBEDTLS_OID_X509_EXT_CERTIFICATE_POLICIES` to the list
of supported x509 extensions, in `mbedtls_oid_get_x509_ext_type()`.
diff --git a/ChangeLog b/ChangeLog
index 9ab0c03..984de6a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -8,6 +8,7 @@
    * It is now possible to use NIST key wrap mode via the mbedtls_cipher API.
      Contributed by Jack Lloyd and Fortanix Inc.
    * Add the Wi-SUN Field Area Network (FAN) device extended key usage.
+   * Add the oid certificate policy x509 extension.
 
 Bugfix
    * Fix private key DER output in the key_app_writer example. File contents