ox-org: Add footnote prefix

* lisp/ox-org.el (org-org-section): Add mandatory "fn" prefix to
  footnotes.

Reported-by: Nick Dokos <ndokos@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/107366>

lisp/ox-org.el

@@ -210,7 +210,7 @@ a communication channel."
 		  (mapconcat
 		   (lambda (d)
 		     (org-element-normalize-string
-		      (concat (format "[%s] "(car d))
+		      (concat (format "[fn:%s] "(car d))
 			      (org-export-data (cdr d) info))))
 		   footnotes-alist "\n"))))
    (make-string (or (org-element-property :post-blank section) 0) ?\n)))