basic + fundamental: Clean up includes

Split out of #37344.

src/basic/ordered-set.c

@@ -1,5 +1,9 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
 #include "fileio.h"
 #include "ordered-set.h"
 #include "strv.h"