Copter: get_rangefinder_height_interpolated_cm made const

ArduCopter/Copter.h

@@ -256,10 +256,8 @@ private:
         uint32_t glitch_cleared_ms; // system time glitch cleared
     } rangefinder_state, rangefinder_up_state;
 
-    /*
+    // return rangefinder height interpolated using inertial altitude
-      return rangefinder height interpolated using inertial altitude
+    bool get_rangefinder_height_interpolated_cm(int32_t& ret) const;
-     */
-    bool get_rangefinder_height_interpolated_cm(int32_t& ret);
 
     class SurfaceTracking {
     public:

ArduCopter/sensors.cpp

@@ -135,7 +135,7 @@ bool Copter::rangefinder_up_ok() const
   difference between the inertial height at that time and the current
   inertial height to give us interpolation of height from rangefinder
  */
-bool Copter::get_rangefinder_height_interpolated_cm(int32_t& ret)
+bool Copter::get_rangefinder_height_interpolated_cm(int32_t& ret) const
 {
     if (!rangefinder_alt_ok()) {
         return false;