These source files uses symbols provided by sched.h, e.g.
setns(), unshare(), CLONE_NEWNS, and friends, but they do not explicitly
include sched.h. Currently, it is included indirectly via missing_syscall.h,
which is included by e.g. pidfd-util.h.
Let's explicitly include headers that provides symbols used in the code.
This is similar to
4f18ff2e29b8054f30b084abcabf5f689f4b340b, but for sched.h.
/* SPDX-License-Identifier: LGPL-2.1-or-later */
#include <fcntl.h>
+#include <sched.h>
#include <sys/ioctl.h>
#include <sys/mount.h>
#include <unistd.h>
#include <linux/veth.h>
#include <net/if.h>
#include <poll.h>
+#include <sched.h>
#include <sys/eventfd.h>
#include <sys/ioctl.h>
#include <sys/stat.h>