/usr/include/xenomai/nucleus/ is in libxenomai-dev 2.6.4+dfsg-0.2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/xenomai/nucleus/assert.h
/usr/include/xenomai/nucleus/bheap.h
/usr/include/xenomai/nucleus/bufd.h
/usr/include/xenomai/nucleus/compiler.h
/usr/include/xenomai/nucleus/heap.h
/usr/include/xenomai/nucleus/hostrt.h
/usr/include/xenomai/nucleus/intr.h
/usr/include/xenomai/nucleus/jhash.h
/usr/include/xenomai/nucleus/map.h
/usr/include/xenomai/nucleus/module.h
/usr/include/xenomai/nucleus/pipe.h
/usr/include/xenomai/nucleus/pod.h
/usr/include/xenomai/nucleus/ppd.h
/usr/include/xenomai/nucleus/queue.h
/usr/include/xenomai/nucleus/registry.h
/usr/include/xenomai/nucleus/sched-idle.h
/usr/include/xenomai/nucleus/sched-rt.h
/usr/include/xenomai/nucleus/sched-sporadic.h
/usr/include/xenomai/nucleus/sched-tp.h
/usr/include/xenomai/nucleus/sched.h
/usr/include/xenomai/nucleus/schedparam.h
/usr/include/xenomai/nucleus/schedqueue.h
/usr/include/xenomai/nucleus/select.h
/usr/include/xenomai/nucleus/seqlock.h
/usr/include/xenomai/nucleus/shadow.h
/usr/include/xenomai/nucleus/stat.h
/usr/include/xenomai/nucleus/synch.h
/usr/include/xenomai/nucleus/sys_ppd.h
/usr/include/xenomai/nucleus/system.h
/usr/include/xenomai/nucleus/thread.h
/usr/include/xenomai/nucleus/timebase.h
/usr/include/xenomai/nucleus/timer.h
/usr/include/xenomai/nucleus/trace.h
/usr/include/xenomai/nucleus/types.h
/usr/include/xenomai/nucleus/vdso.h
/usr/include/xenomai/nucleus/version.h
/usr/include/xenomai/nucleus/vfile.h
/usr/include/xenomai/nucleus/xenomai.h