/usr/include/xfce4/libxfcegui4/ is in libxfcegui4-dev 4.8.1-5ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/xfce4/libxfcegui4/dialogs.h
/usr/include/xfce4/libxfcegui4/gtk_style.h
/usr/include/xfce4/libxfcegui4/gtktoxevent.h
/usr/include/xfce4/libxfcegui4/gui-enum-types.h
/usr/include/xfce4/libxfcegui4/icons.h
/usr/include/xfce4/libxfcegui4/libnetk.h
/usr/include/xfce4/libxfcegui4/libxfcegui4-config.h
/usr/include/xfce4/libxfcegui4/libxfcegui4.h
/usr/include/xfce4/libxfcegui4/netk-application.h
/usr/include/xfce4/libxfcegui4/netk-class-group.h
/usr/include/xfce4/libxfcegui4/netk-enum-types.h
/usr/include/xfce4/libxfcegui4/netk-pager.h
/usr/include/xfce4/libxfcegui4/netk-screen.h
/usr/include/xfce4/libxfcegui4/netk-tasklist.h
/usr/include/xfce4/libxfcegui4/netk-trayicon.h
/usr/include/xfce4/libxfcegui4/netk-util.h
/usr/include/xfce4/libxfcegui4/netk-window-action-menu.h
/usr/include/xfce4/libxfcegui4/netk-window-menu.h
/usr/include/xfce4/libxfcegui4/netk-window.h
/usr/include/xfce4/libxfcegui4/netk-workspace.h
/usr/include/xfce4/libxfcegui4/preview_filesel.h
/usr/include/xfce4/libxfcegui4/session-client.h
/usr/include/xfce4/libxfcegui4/xfce-appmenuitem.h
/usr/include/xfce4/libxfcegui4/xfce-colorbutton.h
/usr/include/xfce4/libxfcegui4/xfce-exec.h
/usr/include/xfce4/libxfcegui4/xfce-filechooser.h
/usr/include/xfce4/libxfcegui4/xfce-gdk-extensions.h
/usr/include/xfce4/libxfcegui4/xfce-gtk-extensions.h
/usr/include/xfce4/libxfcegui4/xfce-heading.h
/usr/include/xfce4/libxfcegui4/xfce-icontheme.h
/usr/include/xfce4/libxfcegui4/xfce-startup-notification.h
/usr/include/xfce4/libxfcegui4/xfce-titled-dialog.h
/usr/include/xfce4/libxfcegui4/xfce-widget-helpers.h
/usr/include/xfce4/libxfcegui4/xfce_aboutdialog.h
/usr/include/xfce4/libxfcegui4/xfce_clock.h
/usr/include/xfce4/libxfcegui4/xfce_decorbutton.h
/usr/include/xfce4/libxfcegui4/xfce_decortoggle.h
/usr/include/xfce4/libxfcegui4/xfce_framebox.h
/usr/include/xfce4/libxfcegui4/xfce_iconbutton.h
/usr/include/xfce4/libxfcegui4/xfce_menubutton.h
/usr/include/xfce4/libxfcegui4/xfce_movehandler.h
/usr/include/xfce4/libxfcegui4/xfce_scaled_image.h
/usr/include/xfce4/libxfcegui4/xfce_systemtray.h
/usr/include/xfce4/libxfcegui4/xfce_togglebutton.h
/usr/include/xfce4/libxfcegui4/xinerama.h