Windows: add missing defer to Watcher.WatchList (#447)

windows.go

@@ -100,7 +100,7 @@ func (w *Watcher) Remove(name string) error {
 // WatchList returns the directories and files that are being monitered.
 func (w *Watcher) WatchList() []string {
 	w.mu.Lock()
-	w.mu.Unlock()
+	defer w.mu.Unlock()
 
 	entries := make([]string, 0, len(w.watches))
 	for _, entry := range w.watches {