trivial: libfwupd: correct building URI path
trivial: libfwupd: correct building URI path

when run on Windows the following is emitted:

```
changing metadata URI from https://cdn.fwupd.org/downloads/firmware.xml.gz to
https://cdn.fwupd.org/downloads\firmware-02681-stable.xml.gz
```

fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd