basic + fundamental: Clean up includes

Split out of #37344.

src/basic/hash-funcs.c

@@ -1,10 +1,12 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include <stdlib.h>
 #include <string.h>
 #include <sys/sysmacros.h>
 
 #include "hash-funcs.h"
 #include "path-util.h"
+#include "siphash24.h"
 #include "strv.h"
 
 void string_hash_func(const char *p, struct siphash *state) {