AC_PID: Change Private members to Protected so that AC_HELI_PID can access them.

libraries/AC_PID/AC_PID.h

@@ -108,7 +108,7 @@ public:
 
     static const struct AP_Param::GroupInfo        var_info[];
 
-private:
+protected:
     AP_Float        _kp;
     AP_Float        _ki;
     AP_Float        _kd;