AP_GPS: make setHIL take const references

libraries/AP_GPS/AP_GPS.cpp

@@ -330,7 +330,6 @@ AP_GPS::update_instance(uint8_t instance)
 void
 AP_GPS::update(void)
 {
-
     for (uint8_t i=0; i<GPS_MAX_INSTANCES; i++) {
         update_instance(i);
     }
@@ -376,7 +375,7 @@ AP_GPS::update(void)
  */
 void 
 AP_GPS::setHIL(uint8_t instance, GPS_Status _status, uint64_t time_epoch_ms, 
-               Location &_location, Vector3f &_velocity, uint8_t _num_sats, 
+               const Location &_location, const Vector3f &_velocity, uint8_t _num_sats, 
                uint16_t hdop, bool _have_vertical_velocity)
 {
     if (instance >= GPS_MAX_INSTANCES) {

libraries/AP_GPS/AP_GPS.h

@@ -278,7 +278,7 @@ public:
 
     // set position for HIL
     void setHIL(uint8_t instance, GPS_Status status, uint64_t time_epoch_ms, 
-                Location &location, Vector3f &velocity, uint8_t num_sats,
+                const Location &location, const Vector3f &velocity, uint8_t num_sats,
                 uint16_t hdop, bool _have_vertical_velocity);
 
     static const struct AP_Param::GroupInfo var_info[];