/usr/include/ is in libdevhelp-dev 3.18.1-1ubuntu5.
This file is owned by root:root, with mode 0o755.
..
/usr/include/devhelp-3.0/
/usr/include/devhelp-3.0/devhelp/
/usr/include/devhelp-3.0/devhelp/dh-book-manager.h
/usr/include/devhelp-3.0/devhelp/dh-language.h
/usr/include/devhelp-3.0/devhelp/devhelp.h
/usr/include/devhelp-3.0/devhelp/dh-book-tree.h
/usr/include/devhelp-3.0/devhelp/dh-book.h
/usr/include/devhelp-3.0/devhelp/dh-error.h
/usr/include/devhelp-3.0/devhelp/dh-link.h
/usr/include/devhelp-3.0/devhelp/dh-window.h
/usr/include/devhelp-3.0/devhelp/dh-sidebar.h
/usr/include/devhelp-3.0/devhelp/dh-keyword-model.h
/usr/include/devhelp-3.0/devhelp/dh-app.h
/usr/include/devhelp-3.0/devhelp/dh-assistant.h
/usr/include/devhelp-3.0/devhelp/dh-assistant-view.h