org-item-re: Convert into defsubst

* lisp/org-list.el (org-item-re): Use faster `defsubst'.  This
function is called very frequently when parsing Org buffers by
`org-element--current-element'.

lisp/org-list.el

@@ -373,7 +373,7 @@ group 4: description tag")
 
 (defvar org--item-re-cache nil
   "Results cache for `org-item-re'.")
-(defun org-item-re ()
+(defsubst org-item-re ()
   "Return the correct regular expression for plain lists."
   (or (plist-get
        (plist-get org--item-re-cache