As Juraj rightly points out, you have a typo, which should give you the following compile time error:
In file included from sketch/Sensor.cpp:1:0:
Sensor.h:17: error: 'pivate' does not name a type
pivate:
^
Here is the correct code for Sensor.h
#ifndef Sensor_h
#define Sensor_h
#include <EEPROM.h>
#include "IMU/L3G.h"
#define CUTOFF_FREQ 30
#define FREQ 400
class Sensor {
public:
Sensor();
~Sensor();
void Init(void);
private:
L3G gyro;
};
#endif