basic + fundamental: Clean up includes

Split out of #37344.

src/basic/syslog-util.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include <stdbool.h>
+#include "forward.h"
 
 int log_facility_unshifted_to_string_alloc(int i, char **s);
 int log_facility_unshifted_from_string(const char *s);