Explicitly enable timestamps in the ALSA backend

Fixes negative values in xrun reports on Linux kernels 4.0 and above.

drivers/alsa/alsa_driver.c

@@ -573,6 +573,11 @@ alsa_driver_configure_stream (alsa_driver_t *driver, char *device_name,
 		return -1;
 	}
 
+	if ((err = snd_pcm_sw_params_set_tstamp_mode(handle, sw_params, SND_PCM_TSTAMP_ENABLE)) < 0) {
+		jack_error("ALSA: cannot set tstamp mode for %s", stream_name);
+		return -1;
+	}
+
 	if ((err = snd_pcm_sw_params (handle, sw_params)) < 0) {
 		jack_error ("ALSA: cannot set software parameters for %s\n",
 			    stream_name);