/usr/include/gtk-3.0/gtk/a11y/ is in libgtk-3-dev 3.18.9-1ubuntu3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/gtk-3.0/gtk/a11y/gtk-a11y-autocleanups.h
/usr/include/gtk-3.0/gtk/a11y/gtkarrowaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkbooleancellaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkbuttonaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkcellaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkcellaccessibleparent.h
/usr/include/gtk-3.0/gtk/a11y/gtkcheckmenuitemaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkcomboboxaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkcontaineraccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkcontainercellaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkentryaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkexpanderaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkflowboxaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkflowboxchildaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkframeaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkiconviewaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkimageaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkimagecellaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtklabelaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtklevelbaraccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtklinkbuttonaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtklistboxaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtklistboxrowaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtklockbuttonaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkmenuaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkmenubuttonaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkmenuitemaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkmenushellaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtknotebookaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtknotebookpageaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkpanedaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkpopoveraccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkprogressbaraccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkradiobuttonaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkradiomenuitemaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkrangeaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkrenderercellaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkscaleaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkscalebuttonaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkscrolledwindowaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkspinbuttonaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkspinneraccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkstatusbaraccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkswitchaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtktextcellaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtktextviewaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtktogglebuttonaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtktoplevelaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtktreeviewaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkwidgetaccessible.h
/usr/include/gtk-3.0/gtk/a11y/gtkwindowaccessible.h