/usr/include/xfce4/ is in xfce4-panel-dev 4.11.0-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/xfce4/libxfce4panel-1.0/
/usr/include/xfce4/libxfce4panel-1.0/libxfce4panel/
/usr/include/xfce4/libxfce4panel-1.0/libxfce4panel/libxfce4panel-config.h
/usr/include/xfce4/libxfce4panel-1.0/libxfce4panel/libxfce4panel-enum-types.h
/usr/include/xfce4/libxfce4panel-1.0/libxfce4panel/libxfce4panel-enums.h
/usr/include/xfce4/libxfce4panel-1.0/libxfce4panel/libxfce4panel.h
/usr/include/xfce4/libxfce4panel-1.0/libxfce4panel/xfce-arrow-button.h
/usr/include/xfce4/libxfce4panel-1.0/libxfce4panel/xfce-hvbox.h
/usr/include/xfce4/libxfce4panel-1.0/libxfce4panel/xfce-panel-convenience.h
/usr/include/xfce4/libxfce4panel-1.0/libxfce4panel/xfce-panel-image.h
/usr/include/xfce4/libxfce4panel-1.0/libxfce4panel/xfce-panel-macros-46.h
/usr/include/xfce4/libxfce4panel-1.0/libxfce4panel/xfce-panel-macros.h
/usr/include/xfce4/libxfce4panel-1.0/libxfce4panel/xfce-panel-plugin-provider.h
/usr/include/xfce4/libxfce4panel-1.0/libxfce4panel/xfce-panel-plugin.h
/usr/include/xfce4/libxfce4panel-2.0/
/usr/include/xfce4/libxfce4panel-2.0/libxfce4panel/
/usr/include/xfce4/libxfce4panel-2.0/libxfce4panel/libxfce4panel-config.h
/usr/include/xfce4/libxfce4panel-2.0/libxfce4panel/libxfce4panel-enum-types.h
/usr/include/xfce4/libxfce4panel-2.0/libxfce4panel/libxfce4panel-enums.h
/usr/include/xfce4/libxfce4panel-2.0/libxfce4panel/libxfce4panel.h
/usr/include/xfce4/libxfce4panel-2.0/libxfce4panel/xfce-arrow-button.h
/usr/include/xfce4/libxfce4panel-2.0/libxfce4panel/xfce-panel-convenience.h
/usr/include/xfce4/libxfce4panel-2.0/libxfce4panel/xfce-panel-image.h
/usr/include/xfce4/libxfce4panel-2.0/libxfce4panel/xfce-panel-macros-46.h
/usr/include/xfce4/libxfce4panel-2.0/libxfce4panel/xfce-panel-macros.h
/usr/include/xfce4/libxfce4panel-2.0/libxfce4panel/xfce-panel-plugin-provider.h
/usr/include/xfce4/libxfce4panel-2.0/libxfce4panel/xfce-panel-plugin.h