Fix dd-agent buildInputs to include pythonPackages.psutil

When deploying current master version journalctl showed ImportErrors in the logs with message:
'No module named psutil' and systemctl dd-agent service failed to start. To fix I added
pythonPackages.psutil as a buildInputs entry and this fixed the issue.

See this Gist for journalctl errors for reference:
https://gist.github.com/mbbx6spp/59eefabbe708c335a31f