/usr/include/xfce4/ is in libxfce4ui-1-dev 4.12.1-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/xfce4/libxfce4kbd-private-2/
/usr/include/xfce4/libxfce4kbd-private-2/libxfce4kbd-private/
/usr/include/xfce4/libxfce4kbd-private-2/libxfce4kbd-private/xfce-shortcut-dialog.h
/usr/include/xfce4/libxfce4kbd-private-2/libxfce4kbd-private/xfce-shortcuts-grabber.h
/usr/include/xfce4/libxfce4kbd-private-2/libxfce4kbd-private/xfce-shortcuts-provider.h
/usr/include/xfce4/libxfce4kbd-private-2/libxfce4kbd-private/xfce-shortcuts-xfwm4.h
/usr/include/xfce4/libxfce4kbd-private-2/libxfce4kbd-private/xfce-shortcuts.h
/usr/include/xfce4/libxfce4ui-1/
/usr/include/xfce4/libxfce4ui-1/libxfce4ui/
/usr/include/xfce4/libxfce4ui-1/libxfce4ui/libxfce4ui-config.h
/usr/include/xfce4/libxfce4ui-1/libxfce4ui/libxfce4ui-enum-types.h
/usr/include/xfce4/libxfce4ui-1/libxfce4ui/libxfce4ui.h
/usr/include/xfce4/libxfce4ui-1/libxfce4ui/xfce-dialogs.h
/usr/include/xfce4/libxfce4ui-1/libxfce4ui/xfce-gdk-extensions.h
/usr/include/xfce4/libxfce4ui-1/libxfce4ui/xfce-gtk-extensions.h
/usr/include/xfce4/libxfce4ui-1/libxfce4ui/xfce-sm-client.h
/usr/include/xfce4/libxfce4ui-1/libxfce4ui/xfce-spawn.h
/usr/include/xfce4/libxfce4ui-1/libxfce4ui/xfce-titled-dialog.h