shared: Clean up includes

Split out of #37344.

src/shared/mount-setup.h

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