Fix include path in the Doxygen output. (#316)
Fix include path in the Doxygen output. (#316)

The Doxygen output was not including the full path of the headers.
The name of the option to enable this is counterintuitive:

https://stackoverflow.com/questions/2422755/how-can-i-force-doxygen-to-show-full-include-path
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp