Avoid unused-parameter warning

tests/api.c

@@ -572,6 +572,8 @@ static int
 _test_espeak_ng_phoneme_events_cb(short *samples, int num_samples, espeak_EVENT *events) {
 	char *out = events->user_data;
 	size_t offset;
+	(void) samples;
+	(void) num_samples;
 	for (espeak_EVENT *e = events; e->type != 0; e++) {
 		if (e->type == espeakEVENT_PHONEME) {
 			if (out[0] == 0) offset = 0;