namespace-util: include sched.h

To make CLONE_NEWNS and friends defined, which is used by clone_flag_to_namespace_type().

src/basic/namespace-util.h

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <sched.h>              /* IWYU pragma: export */
+
 #include "forward.h"
 
 typedef enum NamespaceType {