/usr/include/x86_64-linux-musl/sys/ is in musl-dev 1.1.19-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/x86_64-linux-musl/sys/acct.h
/usr/include/x86_64-linux-musl/sys/auxv.h
/usr/include/x86_64-linux-musl/sys/cachectl.h
/usr/include/x86_64-linux-musl/sys/dir.h
/usr/include/x86_64-linux-musl/sys/epoll.h
/usr/include/x86_64-linux-musl/sys/errno.h
/usr/include/x86_64-linux-musl/sys/eventfd.h
/usr/include/x86_64-linux-musl/sys/fanotify.h
/usr/include/x86_64-linux-musl/sys/fcntl.h
/usr/include/x86_64-linux-musl/sys/file.h
/usr/include/x86_64-linux-musl/sys/fsuid.h
/usr/include/x86_64-linux-musl/sys/inotify.h
/usr/include/x86_64-linux-musl/sys/io.h
/usr/include/x86_64-linux-musl/sys/ioctl.h
/usr/include/x86_64-linux-musl/sys/ipc.h
/usr/include/x86_64-linux-musl/sys/kd.h
/usr/include/x86_64-linux-musl/sys/klog.h
/usr/include/x86_64-linux-musl/sys/mman.h
/usr/include/x86_64-linux-musl/sys/mount.h
/usr/include/x86_64-linux-musl/sys/msg.h
/usr/include/x86_64-linux-musl/sys/mtio.h
/usr/include/x86_64-linux-musl/sys/param.h
/usr/include/x86_64-linux-musl/sys/personality.h
/usr/include/x86_64-linux-musl/sys/poll.h
/usr/include/x86_64-linux-musl/sys/prctl.h
/usr/include/x86_64-linux-musl/sys/procfs.h
/usr/include/x86_64-linux-musl/sys/ptrace.h
/usr/include/x86_64-linux-musl/sys/quota.h
/usr/include/x86_64-linux-musl/sys/reboot.h
/usr/include/x86_64-linux-musl/sys/reg.h
/usr/include/x86_64-linux-musl/sys/resource.h
/usr/include/x86_64-linux-musl/sys/select.h
/usr/include/x86_64-linux-musl/sys/sem.h
/usr/include/x86_64-linux-musl/sys/sendfile.h
/usr/include/x86_64-linux-musl/sys/shm.h
/usr/include/x86_64-linux-musl/sys/signal.h
/usr/include/x86_64-linux-musl/sys/signalfd.h
/usr/include/x86_64-linux-musl/sys/socket.h
/usr/include/x86_64-linux-musl/sys/soundcard.h
/usr/include/x86_64-linux-musl/sys/stat.h
/usr/include/x86_64-linux-musl/sys/statfs.h
/usr/include/x86_64-linux-musl/sys/statvfs.h
/usr/include/x86_64-linux-musl/sys/stropts.h
/usr/include/x86_64-linux-musl/sys/swap.h
/usr/include/x86_64-linux-musl/sys/syscall.h
/usr/include/x86_64-linux-musl/sys/sysinfo.h
/usr/include/x86_64-linux-musl/sys/syslog.h
/usr/include/x86_64-linux-musl/sys/sysmacros.h
/usr/include/x86_64-linux-musl/sys/termios.h
/usr/include/x86_64-linux-musl/sys/time.h
/usr/include/x86_64-linux-musl/sys/timeb.h
/usr/include/x86_64-linux-musl/sys/timerfd.h
/usr/include/x86_64-linux-musl/sys/times.h
/usr/include/x86_64-linux-musl/sys/timex.h
/usr/include/x86_64-linux-musl/sys/ttydefaults.h
/usr/include/x86_64-linux-musl/sys/types.h
/usr/include/x86_64-linux-musl/sys/ucontext.h
/usr/include/x86_64-linux-musl/sys/uio.h
/usr/include/x86_64-linux-musl/sys/un.h
/usr/include/x86_64-linux-musl/sys/user.h
/usr/include/x86_64-linux-musl/sys/utsname.h
/usr/include/x86_64-linux-musl/sys/vfs.h
/usr/include/x86_64-linux-musl/sys/vt.h
/usr/include/x86_64-linux-musl/sys/wait.h
/usr/include/x86_64-linux-musl/sys/xattr.h