org-agenda: convert category to string

* org-agenda.el (org-format-agenda-item): Convert category to a string
if it is a symbol. This fixes the following call to
org-agenda-get-category-icon which fails if category is not a string.

Signed-off-by: Julien Danjou <julien@danjou.info>

lisp/org-agenda.el

@@ -5044,7 +5044,9 @@ Any match of REMOVE-RE will be removed from TXT."
 	       org-agenda-show-inherited-tags
 	       org-agenda-hide-tags-regexp))
     (let* ((category (or category
-			 org-category
+			 (if (stringp org-category)
+			     org-category
+			   (symbol-name org-category))
 			 (if buffer-file-name
 			     (file-name-sans-extension
 			      (file-name-nondirectory buffer-file-name))