namespace-util,nsresource: explicitly include sched.h

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 4f18ff2e29, but for sched.h.

src/basic/namespace-util.c

@@ -1,6 +1,7 @@
 /* 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>