Fix "Not at a block" error on special blocks

* lisp/org.el (org-hide-block-toggle): Fix "Not at a block" error on
  special blocks.

Thanks to Fabrice Popineau for reporting it.
http://permalink.gmane.org/gmane.emacs.orgmode/90663

lisp/org.el

@@ -7340,7 +7340,7 @@ is non-nil, hide it unconditionally."
   (let ((element (org-element-at-point)))
     (unless (memq (org-element-type element)
 		  '(center-block comment-block example-block quote-block
-				 src-block verse-block))
+				 special-block src-block verse-block))
       (user-error "Not at a block"))
     (let* ((start (save-excursion
 		    (goto-char (org-element-property :post-affiliated element))