/usr/include/sys/sendfile.h is in libc6-dev-i386 2.15-0ubuntu10.18.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/include/sys/../x86_64-linux-gnu/sys/sendfile.h
/usr/include/sys/../x86_64-linux-gnu/sys/sendfile.h