AP_ADSB: fix getting a copy rather than a reference

These have been caught by making constructors private and enforcing
calling a create() method to create objects are supposed to be
statically allocated only.

The Vector3f reference was just nearby and was a change copied from
similar PR from Peter Barker (#6873).

libraries/AP_ADSB/AP_ADSB.cpp

@@ -462,8 +462,8 @@ void AP_ADSB::send_adsb_vehicle(const mavlink_channel_t chan)
 
 void AP_ADSB::send_dynamic_out(const mavlink_channel_t chan)
 {
-    AP_GPS gps = _ahrs.get_gps();
-    Vector3f gps_velocity = gps.velocity();
+    const AP_GPS &gps = _ahrs.get_gps();
+    const Vector3f &gps_velocity = gps.velocity();
 
     int32_t latitude = _my_loc.lat;
     int32_t longitude = _my_loc.lng;
@@ -506,7 +506,7 @@ void AP_ADSB::send_dynamic_out(const mavlink_channel_t chan)
     const uint64_t gps_time = gps.time_epoch_usec();
     const uint32_t utcTime = gps_time / 1000000ULL;
 
-    AP_Baro baro = _ahrs.get_baro();
+    const AP_Baro &baro = _ahrs.get_baro();
     int32_t altPres = INT_MAX;
     if (baro.healthy()) {
         // Altitude difference between 101325 (Pascals) and current pressure. Result in millimeters
@@ -578,7 +578,7 @@ uint32_t AP_ADSB::genICAO(const Location_Class &loc)
 {
     // gps_time is not seconds since UTC midnight, but it is an equivalent random number
     // TODO: use UTC time instead of GPS time
-    AP_GPS gps = _ahrs.get_gps();
+    const AP_GPS &gps = _ahrs.get_gps();
     const uint64_t gps_time = gps.time_epoch_usec();
 
     uint32_t timeSum = 0;