Add support for NetBSD

[iamleot]

When trying to build fake-gcs-server on NetBSD it fails with:

# github.com/fsouza/fake-gcs-server/internal/backend
internal/backend/fs.go:120:82: undefined: createTimeFromFileInfo
internal/backend/fs.go:156:82: undefined: createTimeFromFileInfo

This PR addresses that and now fake-gcs-server builds on NetBSD too.

(I have noticed that while updating Grafana Loki.)

[fsouza]

Thanks for contributing!

[iamleot]

Thank you!