undeclared identifier: 'data' when using setCallback template

[benob]

setCallback assumes the existence of a data variable which is passed to the callback: https://github.com/FedericoCeratto/nim-fswatch/blob/60d5190328d019cd53bc7ea58c3252db5036b4c7/fswatch.nim#L110

A temporary fix is to declare a variable named data in the scope where setCallback is called.

var data = ""
monitor.setCallback(callback)