EKF: move small simple getters to header

- return by const reference where possible

EKF/airspeed_fusion.cpp

@@ -161,11 +161,6 @@ void Ekf::fuseAirspeed()
 	}
 }
 
-Vector2f Ekf::getWindVelocity() const
-{
-	return _state.wind_vel;
-}
-
 Vector2f Ekf::getWindVelocityVariance() const
 {
 	return P.slice<2, 2>(22,22).diag();