basic + fundamental: Clean up includes

Split out of #37344.

src/basic/gunicode.h

@@ -6,9 +6,7 @@
  */
 #pragma once
 
-#include <stdbool.h>
-#include <stdint.h>
-#include <stdlib.h>
+#include "forward.h"
 
 char *utf8_prev_char (const char *p);