Plane: fix compile error for RANGEFINDER_ENABLED

Plane: #define check for rangefinder enabled in relative_ground_altitude()

ArduPlane/altitude.cpp

@@ -139,9 +139,11 @@ int32_t Plane::relative_altitude_abs_cm(void)
  */
 float Plane::relative_ground_altitude(bool use_rangefinder_if_available)
 {
+#if RANGEFINDER_ENABLED == ENABLED
    if (use_rangefinder_if_available && rangefinder_state.in_range) {
         return rangefinder_state.height_estimate;
     }
+#endif
 
 #if AP_TERRAIN_AVAILABLE
     float altitude;