fix psk only build fail

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_client.c b/library/ssl_tls13_client.c
index cdb41f0..207098c 100644
--- a/library/ssl_tls13_client.c
+++ b/library/ssl_tls13_client.c
@@ -1943,6 +1943,7 @@
 }
 #endif /* MBEDTLS_SSL_TLS1_3_COMPATIBILITY_MODE */
 
+#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
 /*
  * Handler for MBEDTLS_SSL_CLIENT_CERTIFICATE
  */
@@ -1962,6 +1963,7 @@
 {
     return( mbedtls_ssl_tls13_write_certificate_verify( ssl ) );
 }
+#endif /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED */
 
 /*
  * Handler for MBEDTLS_SSL_CLIENT_FINISHED
@@ -2057,6 +2059,7 @@
             ret = ssl_tls13_process_server_finished( ssl );
             break;
 
+#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
         case MBEDTLS_SSL_CLIENT_CERTIFICATE:
             ret = ssl_tls13_write_client_certificate( ssl );
             break;
@@ -2064,7 +2067,7 @@
         case MBEDTLS_SSL_CLIENT_CERTIFICATE_VERIFY:
             ret = ssl_tls13_write_client_certificate_verify( ssl );
             break;
-
+#endif /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED */
 
         case MBEDTLS_SSL_CLIENT_FINISHED:
             ret = ssl_tls13_write_client_finished( ssl );