emacs: Fix to eliminate warning in notmuch-query-map-aux

This is one of those cases where the warning looks absolutely correct,
(complaining about a free variable), but I'm left wondering how the
original code could have worked at all.

From what I can tell, this code wasn't actually being called by any
of the current code in notmuch.

emacs/notmuch-query.el

@@ -47,7 +47,7 @@ is a possibly empty forest of replies.
   (apply 'append
 	 (mapcar
 	   (lambda (tree)
-	     (funcall mapper fn tree))
+	     (funcall mapper function tree))
 	   seq)))
 
 (defun notmuch-query-map-threads (fn threads)