AP_AHRS: fixed functions that need to be virtual

functions overridden in a child class need to be marked virtual, or
you get the parent class function

libraries/AP_AHRS/AP_AHRS.h

@@ -119,7 +119,7 @@ public:
     // dead-reckoning. Return true if a position is available,
     // otherwise false. This only updates the lat and lng fields
     // of the Location
-    bool get_position(struct Location *loc) {
+    virtual bool get_position(struct Location *loc) {
         if (!_gps || _gps->status() <= GPS::NO_FIX) {
             return false;
         }
@@ -129,16 +129,16 @@ public:
     }
 
     // return a wind estimation vector, in m/s
-    Vector3f wind_estimate(void) {
+    virtual Vector3f wind_estimate(void) {
         return Vector3f(0,0,0);
     }
 
     // return an airspeed estimate if available. return true
     // if we have an estimate
-    bool airspeed_estimate(float *airspeed_ret);
+    virtual bool airspeed_estimate(float *airspeed_ret);
 
     // return true if we will use compass for yaw
-    bool use_compass(void) { return _compass && _compass->use_for_yaw(); }
+    virtual bool use_compass(void) { return _compass && _compass->use_for_yaw(); }
 
     // return true if yaw has been initialised
     bool yaw_initialised(void) {