Fix unused variable in some TLS 1.3 builds

Fix unused variable when MBEDTLS_SSL_PROTO_TLS1_3 and
MBEDTLS_SSL_SESSION_TICKETS are enabled but not MBEDTLS_DEBUG_C.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index 20cecdb..81fa514 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -837,6 +837,8 @@
                              ssl, MBEDTLS_SSL_HS_CERTIFICATE, buf, buf_len));
 
 cleanup:
+#else /* MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_EPHEMERAL_ENABLED */
+    (void) ssl;
 #endif /* MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_EPHEMERAL_ENABLED */
 
     MBEDTLS_SSL_DEBUG_MSG(2, ("<= parse certificate"));