ox: Fix "void-variable ignore" error when publishing

* lisp/ox.el (org-export-async-start): Allow to use symbols as
  function.

Thanks to Julien Cubizolles for reporting it.
http://permalink.gmane.org/gmane.emacs.orgmode/91532

lisp/ox.el

@@ -5532,7 +5532,7 @@ and `org-export-to-file' for more specialized functions."
            ;; Register running process in stack.
            (org-export-add-to-stack (get-buffer ,proc-buffer) nil ,process)
            ;; Set-up sentinel in order to catch results.
-           (let ((handler ,fun))
+           (let ((handler ',fun))
              (set-process-sentinel
               ,process
               `(lambda (p status)