AHRS: avoid a compiler bug in quaternion code

Having _wind in the AP_AHRS class causes a register allocation error
when building the Quaternion code with some versions of avr-gcc. Quite
bizarre.

libraries/AP_AHRS/AP_AHRS.h

@@ -104,10 +104,11 @@ public:
 
 	// return a wind estimation vector, in m/s
 	Vector3f wind_estimate(void) {
-		return _wind;
+		return Vector3f(0,0,0);
 	}
 
 protected:
+
 	// pointer to compass object, if available
 	Compass 	* _compass;
 
@@ -131,9 +132,6 @@ protected:
 	// radians/s/s
 	float           _gyro_drift_limit;
 
-	// estimated wind in m/s
-	Vector3f	_wind;
-
 	// acceleration due to gravity in m/s/s
 	static const float _gravity = 9.80665;
 };

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -53,6 +53,11 @@ public:
 	// allow for runtime disabling of GPS usage for position
 	AP_Int8		_gps_use;
 
+	// return a wind estimation vector, in m/s
+	Vector3f wind_estimate(void) {
+		return _wind;
+	}
+
 	// for holding parameters
 	static const struct AP_Param::GroupInfo var_info[];
 
@@ -131,6 +136,9 @@ private:
 	Vector3f	_last_vel;
 	uint32_t	_last_wind_time;
 	float		_last_airspeed;       
+
+	// estimated wind in m/s
+	Vector3f	_wind;
 };
 
 #endif // AP_AHRS_DCM_H