Overall this is not actually deb build specific issue, but something that happens during install, as I could spot during building and installing using git repo inside a docker container (for kamailio unit tests). I guess that was more an optional step, attempting to build man pages from docbooks for modules readmes.
I think this should be made optional properly. I do not think anyone was using manpage-style docs so far for modules and starting now to install/package them all makes no sense.