refactor: remove some unused includes (#19747)

- Remove autocmd.h from fileio.h
- Remove normal.h from main.h
- Move bufinfo_T from undo_defs.h to undo.c

src/nvim/buffer.c

@@ -27,6 +27,7 @@
 #include "nvim/api/private/helpers.h"
 #include "nvim/ascii.h"
 #include "nvim/assert.h"
+#include "nvim/autocmd.h"
 #include "nvim/buffer.h"
 #include "nvim/buffer_updates.h"
 #include "nvim/change.h"