use fsnotify.v0 for WatchFile gadget

fsnotify is currently being maintained at: https://github.com/go-fsnotify/fsnotify

fsnotify.v0 matches the API you are using but some bugs have been fixed. There
is also a newer v1 API if you wish to upgrade.

(So I know when this is merged, ref: https://github.com/go-fsnotify/fsnotify/issues/35)