/usr/include/dlib/gui_widgets/ is in libdlib-dev 18.18-2build1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/dlib/gui_widgets/base_widgets.h
/usr/include/dlib/gui_widgets/base_widgets_abstract.h
/usr/include/dlib/gui_widgets/canvas_drawing.h
/usr/include/dlib/gui_widgets/canvas_drawing_abstract.h
/usr/include/dlib/gui_widgets/drawable.h
/usr/include/dlib/gui_widgets/drawable_abstract.h
/usr/include/dlib/gui_widgets/fonts.h
/usr/include/dlib/gui_widgets/fonts_abstract.h
/usr/include/dlib/gui_widgets/nativefont.h
/usr/include/dlib/gui_widgets/style.h
/usr/include/dlib/gui_widgets/style_abstract.h
/usr/include/dlib/gui_widgets/widgets.h
/usr/include/dlib/gui_widgets/widgets_abstract.h