/usr/lib/x86_64-linux-gnu/ is in libgtk-3-dev 3.22.11-1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/x86_64-linux-gnu/pkgconfig/
/usr/lib/x86_64-linux-gnu/pkgconfig/gdk-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gdk-broadway-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gdk-wayland-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gdk-x11-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-broadway-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-unix-print-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-wayland-3.0.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-x11-3.0.pc
/usr/lib/x86_64-linux-gnu/libgdk-3.so
/usr/lib/x86_64-linux-gnu/libgtk-3.so