/usr/include/x86_64-linux-gnu/nx-X11/ is in libnx-x11-dev 2:3.5.99.16-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/x86_64-linux-gnu/nx-X11/ImUtil.h
/usr/include/x86_64-linux-gnu/nx-X11/XKBlib.h
/usr/include/x86_64-linux-gnu/nx-X11/Xauth.h
/usr/include/x86_64-linux-gnu/nx-X11/Xcms.h
/usr/include/x86_64-linux-gnu/nx-X11/Xlib.h
/usr/include/x86_64-linux-gnu/nx-X11/XlibConf.h
/usr/include/x86_64-linux-gnu/nx-X11/Xlibint.h
/usr/include/x86_64-linux-gnu/nx-X11/Xlocale.h
/usr/include/x86_64-linux-gnu/nx-X11/Xregion.h
/usr/include/x86_64-linux-gnu/nx-X11/Xresource.h
/usr/include/x86_64-linux-gnu/nx-X11/Xutil.h
/usr/include/x86_64-linux-gnu/nx-X11/cursorfont.h
/usr/include/x86_64-linux-gnu/nx-X11/misc.h
/usr/include/x86_64-linux-gnu/nx-X11/os.h