refactor: iwyu #31637

Result of `make iwyu` (after some "fixups").

src/nvim/mbyte.c

@@ -26,6 +26,7 @@
 #include <ctype.h>
 #include <errno.h>
 #include <iconv.h>
+#include <limits.h>
 #include <locale.h>
 #include <stdbool.h>
 #include <stddef.h>
@@ -51,7 +52,6 @@
 #include "nvim/gettext_defs.h"
 #include "nvim/globals.h"
 #include "nvim/grid.h"
-#include "nvim/grid_defs.h"
 #include "nvim/iconv_defs.h"
 #include "nvim/keycodes.h"
 #include "nvim/macros_defs.h"