/usr/include/X11/extensions/ is in x11proto-xext-dev 7.2.0-3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/X11/extensions/EVI.h
/usr/include/X11/extensions/EVIproto.h
/usr/include/X11/extensions/ag.h
/usr/include/X11/extensions/agproto.h
/usr/include/X11/extensions/cup.h
/usr/include/X11/extensions/cupproto.h
/usr/include/X11/extensions/dbe.h
/usr/include/X11/extensions/dbeproto.h
/usr/include/X11/extensions/dpmsconst.h
/usr/include/X11/extensions/dpmsproto.h
/usr/include/X11/extensions/ge.h
/usr/include/X11/extensions/geproto.h
/usr/include/X11/extensions/lbx.h
/usr/include/X11/extensions/lbxproto.h
/usr/include/X11/extensions/mitmiscconst.h
/usr/include/X11/extensions/mitmiscproto.h
/usr/include/X11/extensions/multibufconst.h
/usr/include/X11/extensions/multibufproto.h
/usr/include/X11/extensions/secur.h
/usr/include/X11/extensions/securproto.h
/usr/include/X11/extensions/shapeconst.h
/usr/include/X11/extensions/shapeproto.h
/usr/include/X11/extensions/shapestr.h
/usr/include/X11/extensions/shm.h
/usr/include/X11/extensions/shmproto.h
/usr/include/X11/extensions/shmstr.h
/usr/include/X11/extensions/syncconst.h
/usr/include/X11/extensions/syncproto.h
/usr/include/X11/extensions/syncstr.h
/usr/include/X11/extensions/xtestconst.h
/usr/include/X11/extensions/xtestext1const.h
/usr/include/X11/extensions/xtestext1proto.h
/usr/include/X11/extensions/xtestproto.h