/usr/include/gdkmm-2.4/gdkmm/private/ is in libgtkmm-2.4-dev 1:2.24.5-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/gdkmm-2.4/gdkmm/private/bitmap_p.h
/usr/include/gdkmm-2.4/gdkmm/private/color_p.h
/usr/include/gdkmm-2.4/gdkmm/private/colormap_p.h
/usr/include/gdkmm-2.4/gdkmm/private/cursor_p.h
/usr/include/gdkmm-2.4/gdkmm/private/device_p.h
/usr/include/gdkmm-2.4/gdkmm/private/display_p.h
/usr/include/gdkmm-2.4/gdkmm/private/displaymanager_p.h
/usr/include/gdkmm-2.4/gdkmm/private/dragcontext_p.h
/usr/include/gdkmm-2.4/gdkmm/private/drawable_p.h
/usr/include/gdkmm-2.4/gdkmm/private/event_p.h
/usr/include/gdkmm-2.4/gdkmm/private/gc_p.h
/usr/include/gdkmm-2.4/gdkmm/private/image_p.h
/usr/include/gdkmm-2.4/gdkmm/private/pixbuf_p.h
/usr/include/gdkmm-2.4/gdkmm/private/pixbufanimation_p.h
/usr/include/gdkmm-2.4/gdkmm/private/pixbufanimationiter_p.h
/usr/include/gdkmm-2.4/gdkmm/private/pixbufformat_p.h
/usr/include/gdkmm-2.4/gdkmm/private/pixbufloader_p.h
/usr/include/gdkmm-2.4/gdkmm/private/pixmap_p.h
/usr/include/gdkmm-2.4/gdkmm/private/rectangle_p.h
/usr/include/gdkmm-2.4/gdkmm/private/region_p.h
/usr/include/gdkmm-2.4/gdkmm/private/rgbcmap_p.h
/usr/include/gdkmm-2.4/gdkmm/private/screen_p.h
/usr/include/gdkmm-2.4/gdkmm/private/types_p.h
/usr/include/gdkmm-2.4/gdkmm/private/visual_p.h
/usr/include/gdkmm-2.4/gdkmm/private/window_p.h