Fix example mcp4725_keypad.ino; does not compile for ESP32

[RobTillaart]

I2CKeyPad .cpp:20:6: error: 'PCF8574' has not been declared

void PCF8574::begin(uint8_t sda, uint8_t scl, uint8_t address)

So problem could be in KeyPad library ...

[RobTillaart]

• Example compiles for the UNO
• investigate keypad library ==> has same bug
• solved bug in I2CKeypad 0.1.1 version
• example here does compile again => solved.