/usr/include/X11/extensions/ is in libxext-dev 2:1.3.3-1+b2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/X11/extensions/MITMisc.h
/usr/include/X11/extensions/XEVI.h
/usr/include/X11/extensions/XLbx.h
/usr/include/X11/extensions/XShm.h
/usr/include/X11/extensions/Xag.h
/usr/include/X11/extensions/Xcup.h
/usr/include/X11/extensions/Xdbe.h
/usr/include/X11/extensions/Xext.h
/usr/include/X11/extensions/Xge.h
/usr/include/X11/extensions/dpms.h
/usr/include/X11/extensions/extutil.h
/usr/include/X11/extensions/multibuf.h
/usr/include/X11/extensions/security.h
/usr/include/X11/extensions/shape.h
/usr/include/X11/extensions/sync.h
/usr/include/X11/extensions/xtestext1.h