Revert "namespace-util: include sched.h"

This reverts commit 50cda6433b.

Same as the previous revert, sched.h is too generic to
be included in namespace-util.h, plus this defeats
the purpose of forward decl.

src/basic/namespace-util.h

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