basic + fundamental: Clean up includes

Split out of #37344.

src/basic/pthread-util.h

@@ -1,9 +1,9 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include <pthread.h>
+#include <pthread.h> /* IWYU pragma: export */
 
-#include "assert-util.h"
+#include "forward.h"
 
 static inline pthread_mutex_t* pthread_mutex_lock_assert(pthread_mutex_t *mutex) {
         assert_se(pthread_mutex_lock(mutex) == 0);