Make endpoint getter parameter a pointer to const

It would be convenient for users to query the endpoint
type directly from a ssl context:

```
    mbedtls_ssl_conf_get_endpoint(
        mbedtls_ssl_context_get_config(&ssl))
```

Signed-off-by: Pengyu Lv <pengyu.lv@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 4d86525..7b11e51 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1926,7 +1926,7 @@
  * \return         Endpoint type, either MBEDTLS_SSL_IS_CLIENT
  *                 or MBEDTLS_SSL_IS_SERVER
  */
-static inline int mbedtls_ssl_conf_get_endpoint(mbedtls_ssl_config *conf)
+static inline int mbedtls_ssl_conf_get_endpoint(const mbedtls_ssl_config *conf)
 {
     return conf->MBEDTLS_PRIVATE(endpoint);
 }