tls13: guards transform negotiate

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 3f48377..b8e739b 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1656,9 +1656,11 @@
     mbedtls_ssl_transform *MBEDTLS_PRIVATE(transform);           /*!<  negotiated transform params
                                                                   *    This pointer owns the transform
                                                                   *    it references.                  */
+#if defined(MBEDTLS_SSL_PROTO_TLS1_2)
     mbedtls_ssl_transform *MBEDTLS_PRIVATE(transform_negotiate); /*!<  transform params in negotiation
                                                                   *    This pointer owns the transform
                                                                   *    it references.                  */
+#endif /* MBEDTLS_SSL_PROTO_TLS1_2 */
 
 #if defined(MBEDTLS_SSL_PROTO_TLS1_3)
     /*! The application data transform in TLS 1.3.