shared: Clean up includes

Split out of #37344.

src/shared/securebits-util.h

@@ -1,9 +1,9 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include <errno.h>
 #include <linux/securebits.h>
-#include <stdbool.h>
+
+#include "forward.h"
 
 int secure_bits_to_string_alloc(int i, char **s);
 int secure_bits_from_string(const char *s);