shared: Clean up includes

Split out of #37344.

src/shared/reboot-util.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include <stdbool.h>
+#include "forward.h"
 
 bool reboot_parameter_is_valid(const char *parameter);
 int update_reboot_parameter_and_warn(const char *parameter, bool keep);