GPS simulation: Manage delays correctly

The GPS simulation now mimicks the real driver more closely and should provide even GPS delays. The delays themselves are set by the simulator, and default to 120 ms for Gazebo

src/modules/simulator/simulator_mavlink.cpp

@@ -261,6 +261,7 @@ void Simulator::update_sensors(mavlink_hil_sensor_t *imu)
 void Simulator::update_gps(mavlink_hil_gps_t *gps_sim)
 {
 	RawGPSData gps = {};
+	gps.timestamp = gps_sim->time_usec;
 	gps.lat = gps_sim->lat;
 	gps.lon = gps_sim->lon;
 	gps.alt = gps_sim->alt;