Guard ticket specific TLS 1.3 function with macro

Guard ssl_tls13_write_new_session_ticket_coordinate with
MBEDTLS_SSL_SESSION_TICKETS macro.

Signed-off-by: Norbert Fabritius <norbert.fabritius@esrlabs.com>
diff --git a/library/ssl_tls13_server.c b/library/ssl_tls13_server.c
index 2760d76..c0d1f8d 100644
--- a/library/ssl_tls13_server.c
+++ b/library/ssl_tls13_server.c
@@ -3109,6 +3109,7 @@
     return 0;
 }
 
+#if defined(MBEDTLS_SSL_SESSION_TICKETS)
 /*
  * Handler for MBEDTLS_SSL_TLS1_3_NEW_SESSION_TICKET
  */
@@ -3138,7 +3139,6 @@
     return SSL_NEW_SESSION_TICKET_WRITE;
 }
 
-#if defined(MBEDTLS_SSL_SESSION_TICKETS)
 MBEDTLS_CHECK_RETURN_CRITICAL
 static int ssl_tls13_prepare_new_session_ticket(mbedtls_ssl_context *ssl,
                                                 unsigned char *ticket_nonce,