/usr/include/xenomai/posix/ is in libxenomai-dev 2.6.4+dfsg-0.2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/xenomai/posix/errno.h
/usr/include/xenomai/posix/fcntl.h
/usr/include/xenomai/posix/mqueue.h
/usr/include/xenomai/posix/posix.h
/usr/include/xenomai/posix/pthread.h
/usr/include/xenomai/posix/sched.h
/usr/include/xenomai/posix/semaphore.h
/usr/include/xenomai/posix/signal.h
/usr/include/xenomai/posix/stdio.h
/usr/include/xenomai/posix/sys/
/usr/include/xenomai/posix/sys/ioctl.h
/usr/include/xenomai/posix/sys/mman.h
/usr/include/xenomai/posix/sys/select.h
/usr/include/xenomai/posix/sys/socket.h
/usr/include/xenomai/posix/syscall.h
/usr/include/xenomai/posix/syslog.h
/usr/include/xenomai/posix/time.h
/usr/include/xenomai/posix/unistd.h