vendor/go4.org/rollsum: actually create the package dir

I mistakenly added the files directly in vendor/go4.org instead of
vendor/go4.org/rollsum in 2a85cb7142e6af5439d6d9cf2f6ddada04b4797e

Neither running 'go run make.go', nor running 'go test' (in e.g.
pkg/schema) caught that the package was missing in vendor, because i
happened to also have the package in my GOPATH.

Iirc, before modules existed it would have been enough to run 'go run
make.go' to catch that, whereas now 'go run make.go' ran happily for me,
even with GO111MODULE=off, which is somewhat annoying.

Change-Id: I97805c924700fa159a14fd33014dabc423a4fc60
diff --git a/vendor/go4.org/rollsum.go b/vendor/go4.org/rollsum/rollsum.go
similarity index 100%
rename from vendor/go4.org/rollsum.go
rename to vendor/go4.org/rollsum/rollsum.go
diff --git a/vendor/go4.org/rollsum_test.go b/vendor/go4.org/rollsum/rollsum_test.go
similarity index 100%
rename from vendor/go4.org/rollsum_test.go
rename to vendor/go4.org/rollsum/rollsum_test.go