/usr/include/gdkmm-3.0/gdkmm/private/ is in libgtkmm-3.0-dev 3.18.0-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/gdkmm-3.0/gdkmm/private/applaunchcontext_p.h
/usr/include/gdkmm-3.0/gdkmm/private/color_p.h
/usr/include/gdkmm-3.0/gdkmm/private/cursor_p.h
/usr/include/gdkmm-3.0/gdkmm/private/device_p.h
/usr/include/gdkmm-3.0/gdkmm/private/devicemanager_p.h
/usr/include/gdkmm-3.0/gdkmm/private/display_p.h
/usr/include/gdkmm-3.0/gdkmm/private/displaymanager_p.h
/usr/include/gdkmm-3.0/gdkmm/private/dragcontext_p.h
/usr/include/gdkmm-3.0/gdkmm/private/event_p.h
/usr/include/gdkmm-3.0/gdkmm/private/glcontext_p.h
/usr/include/gdkmm-3.0/gdkmm/private/pixbuf_p.h
/usr/include/gdkmm-3.0/gdkmm/private/pixbufanimation_p.h
/usr/include/gdkmm-3.0/gdkmm/private/pixbufanimationiter_p.h
/usr/include/gdkmm-3.0/gdkmm/private/pixbufformat_p.h
/usr/include/gdkmm-3.0/gdkmm/private/pixbufloader_p.h
/usr/include/gdkmm-3.0/gdkmm/private/rectangle_p.h
/usr/include/gdkmm-3.0/gdkmm/private/rgba_p.h
/usr/include/gdkmm-3.0/gdkmm/private/screen_p.h
/usr/include/gdkmm-3.0/gdkmm/private/timecoord_p.h
/usr/include/gdkmm-3.0/gdkmm/private/types_p.h
/usr/include/gdkmm-3.0/gdkmm/private/visual_p.h
/usr/include/gdkmm-3.0/gdkmm/private/window_p.h