tree-wide: Make sure all headers are standalone

Let's make sure all of our headers include everything they require.

src/nspawn/nspawn-mount.h

@@ -2,6 +2,8 @@
 #pragma once
 
 #include <stdbool.h>
+#include <stddef.h>
+#include <sys/types.h>
 
 #include "volatile-util.h"
 

src/nspawn/nspawn-seccomp.h

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include <sys/types.h>
+#include <stdint.h>
 
 int setup_seccomp(uint64_t cap_list_retain, char **syscall_allow_list, char **syscall_deny_list);

src/nspawn/nspawn-setuid.h

@@ -1,5 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <stdbool.h>
+#include <sys/types.h>
+
 int change_uid_gid_raw(uid_t uid, gid_t gid, const gid_t *supplementary_gids, size_t n_supplementary_gids, bool chown_stdio);
 int change_uid_gid(const char *user, bool chown_stdio, char **ret_home);