AP_Compass: fix mismatched tag

In file included from /home/lucas/p/dronecode/ardupilot/libraries/AP_Compass/Compass.h:6:
/home/lucas/p/dronecode/ardupilot/libraries/AP_Compass/CompassCalibrator.h:86:5: warning: struct 'param_t' was previously declared as a class [-Wmismatched-tags]
    struct param_t _params;
    ^
/home/lucas/p/dronecode/ardupilot/libraries/AP_Compass/CompassCalibrator.h:44:11: note: previous use is here
    class param_t {
          ^
/home/lucas/p/dronecode/ardupilot/libraries/AP_Compass/CompassCalibrator.h:86:5: note: did you mean class here?
    struct param_t _params;
    ^~~~~~

libraries/AP_Compass/CompassCalibrator.h

@@ -83,7 +83,7 @@ private:
     uint8_t _attempt;
 
     //fit state
-    struct param_t _params;
+    class param_t _params;
     uint16_t _fit_step;
     CompassSample *_sample_buffer;
     float _fitness; // mean squared residuals