JACK backend allows to calibrate latency

However, if jackd is running, we should test if jackd is using
a full-duplex device to establish if latency calibration is
possible -- fix 92f54b3e98.

libs/backends/jack/jack_audiobackend.h

@@ -191,7 +191,7 @@ class JACKAudioBackend : public AudioBackend {
 	return true;
     }
     bool can_measure_systemic_latency () const {
-	return false;
+	return true;
     }
 
     int      midi_event_get (pframes_t& timestamp, size_t& size, uint8_t const** buf, void* port_buffer, uint32_t event_index);