Explicitly mark mbedtls_ssl_config.respect_cli_pref as private

This was always intended to be explicitly marked private.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index e69bdb6..610aa14 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1214,7 +1214,7 @@
 #if defined(MBEDTLS_SSL_SRV_C)
     unsigned int MBEDTLS_PRIVATE(cert_req_ca_list) : 1;  /*!< enable sending CA list in
                                           Certificate Request messages?     */
-    unsigned int respect_cli_pref : 1;  /*!< pick the ciphersuite according to
+    unsigned int MBEDTLS_PRIVATE(respect_cli_pref) : 1;  /*!< pick the ciphersuite according to
                                           the client's preferences rather
                                           than ours                         */
 #endif