Update Ticker.cpp

[yaroshd81]

fix error when call method active(): undefined reference to Ticker::active()

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[bertmelis]

I'm not sure about the behavioural specifics but I don't think this is right.

What should be the result when a one shot timer has already fires?
If your PR it would still be active but I'd say it is not active anymore.

[everslick]

Still it is the same implementation as on ESP8266:

bool Ticker::active() const                                                     
{                                                                               
    return _timer;                                                              
}    

[everslick]

The return type is wrong!!! Should be bool obviously.

[everslick]

we could just return bool esp_timer_is_active(esp_timer_handle_t timer) and call it a day, though.

[me-no-dev]

@everslick want to make a new PR with that? @yaroshd81 seems unresponsive?