sum: Fix tarsum build error

Builds of `d2r` currently fail with:

    $ go get github.com/vbatts/docker-utils/cmd/d2r
    src/github.com/vbatts/docker-utils/sum/layer.go:60: invalid pointer type *tarsum.TarSum for composite literal

It looks like Docker's interface might have changed in docker/docker@ec01eb653db9d5e5b9291c1670dde57e7f742874, and switching to `tarsum.NewTarSum` seems to fix the build error.