org-feed.el: Load the xml library before substituting a guid XML element

* lisp/org-feed.el (org-feed-parse-rss-feed): Load the xml library
before calling `xml-substitute-special`.

TINYCHANGE

lisp/org-feed.el

@@ -604,6 +604,7 @@ Assumes headers are indeed present!"
   "Parse BUFFER for RSS feed entries.
 Returns a list of entries, with each entry a property list,
 containing the properties `:guid' and `:item-full-text'."
+  (require 'xml)
   (let ((case-fold-search t)
 	entries beg end item guid entry)
     (with-current-buffer buffer