knot-resolver-manager_6: include `watchdog`
Upstream it is listed as an optional python dependency, alongside
prometheus-client:
https://github.com/CZ-NIC/knot-resolver/blob/e16aea363f6d2ca39c8a3713f42f21297b72a078/pyproject.toml#L41
It is required when using `network.tls.watchdog=true` in the settings:
https://www.knot-resolver.cz/documentation/latest/config-network-server-tls.html#cmdoption-arg-files-watchdog
```
[ERROR] stderr) Configuration validation error detected:
[ERROR] stderr) [/network/tls] 'files-watchdog' is enabled, but the required 'watchdog' dependency (optional) is not installed
```
