use freestanding headers

boot/cmdline.c

@@ -1,8 +1,8 @@
 #include <zinnia/cmdline.h>
 #include <zinnia/init.h>
 #include <stddef.h>
-#include <stdlib.h>
-#include <string.h>
+#include <zinnia/stdlib.h>
+#include <zinnia/string.h>
 
 // Defined in linker script.
 extern const struct cmdline_option* __ld_cmdline_start[];

boot/limine/entry.c

@@ -3,7 +3,7 @@
 #include <zinnia/init.h>
 #include <zinnia/print.h>
 #include <zinnia/utils.h>
-#include <string.h>
+#include <zinnia/string.h>
 #include "limine.h"
 
 [[__initdata_sorted("limine.0")]]