go-fuse has added a new method to the nodefs.File interface that
caused this build error:
internal/fusefrontend/file.go:75: cannot use file literal (type *file) as type nodefs.File in return argument:
*file does not implement nodefs.File (missing Flock method)
Fixes https://github.com/rfjakob/gocryptfs/issues/104 and
prevents the problem from happening again.