AP_AHRS: make airspeed_estimate const

libraries/AP_AHRS/AP_AHRS.cpp

@@ -117,7 +117,7 @@ const AP_Param::GroupInfo AP_AHRS::var_info[] PROGMEM = {
 };
 
 // return airspeed estimate if available
-bool AP_AHRS::airspeed_estimate(float *airspeed_ret)
+bool AP_AHRS::airspeed_estimate(float *airspeed_ret) const
 {
 	if (_airspeed && _airspeed->use()) {
 		*airspeed_ret = _airspeed->get_airspeed();

libraries/AP_AHRS/AP_AHRS.h

@@ -173,11 +173,11 @@ public:
 
     // return an airspeed estimate if available. return true
     // if we have an estimate
-    virtual bool airspeed_estimate(float *airspeed_ret);
+    virtual bool airspeed_estimate(float *airspeed_ret) const;
 
     // return a true airspeed estimate (navigation airspeed) if
     // available. return true if we have an estimate
-    bool airspeed_estimate_true(float *airspeed_ret) {
+    bool airspeed_estimate_true(float *airspeed_ret) const {
         if (!airspeed_estimate(airspeed_ret)) {
             return false;
         }