KDirWatch: don't try inotify again if it has already failed
If inotify is supported QFileSystemWatcher will use it, so if we already tried useInotify() there is no point trying useQFSWatch() which will just try using inotify internally again.
If inotify is supported QFileSystemWatcher will use it, so if we already tried useInotify() there is no point trying useQFSWatch() which will just try using inotify internally again.