Add server only guards for psk callback

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/tests/suites/test_suite_ssl.function b/tests/suites/test_suite_ssl.function
index f24d1a4..7b5743e 100644
--- a/tests/suites/test_suite_ssl.function
+++ b/tests/suites/test_suite_ssl.function
@@ -2184,8 +2184,9 @@
                              options->psk_str->len,
                              (const unsigned char *) psk_identity,
                              strlen( psk_identity ) ) == 0 );
-
+#if defined(MBEDTLS_SSL_SRV_C)
         mbedtls_ssl_conf_psk_cb( &server.conf, psk_dummy_callback, NULL );
+#endif
     }
 #endif
 #if defined(MBEDTLS_SSL_RENEGOTIATION)