go-proto: fix import errors

The previous update to rootlesscontainers.pb.go caused the package name
of the generated Go file to change, causing the following import errors
when trying to import the go-proto into any other project:

> found packages go_proto (rootlesscontainers.pb.go)
> and rootlesscontainers (rootlesscontainers_generate.go) in
> .../github.com/rootless-containers/proto/go-proto

Running "go generate ./rootlesscontainers_generate.go" solves this
issue.

Fixes: commit 7501d1e ("go-proto: regenerate rootlesscontainers.pb.go")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

Author: cyphar