ob-screen.el: Fix compiler warning

* lisp/ob-screen.el (org-babel-screen-test): Use `message'
instead of `format'.

lisp/ob-screen.el

@@ -126,7 +126,7 @@ The terminal should shortly flicker."
     ;; XXX: need to find a better way to do the following
     (while (not (file-readable-p tmpfile))
       ;; do something, otherwise this will be optimized away
-      (format "org-babel-screen: File not readable yet."))
+      (message "org-babel-screen: File not readable yet."))
     (setq tmp-string (with-temp-buffer
                        (insert-file-contents-literally tmpfile)
                        (buffer-substring (point-min) (point-max))))