Update Ticker.cpp

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

Author: yaroshd81

libraries/Ticker/src/Ticker.cpp

@@ -56,3 +56,7 @@ void Ticker::detach() {
     _timer = nullptr;
   }
 }
+
+void Ticker::active() {
+  return (bool)_timer; 
+}