Docs need the generated agent docs and they are generated at the same time
as the agent sources so not depending on them means, docs fail to build if
they are built before the agent sources:

ERROR: 'gtkdoc-mkhtml' failed with status 6
warning: failed to load external entity "../../src/agent/docs-org.freedesktop.GeoClue2.Agent.xml"
../geoclue-docs.xml:80: element include: XInclude error : could not load ../../src/agent/docs-org.freedesktop.GeoClue2.Agent.xml, and no fallback was found