Fix build with emacs-24.4.

http://debbugs.gnu.org/cgi/bugreport.cgi?bug=18845

--- protobuf-2.5.0/editors/protobuf-mode.el
+++ protobuf-2.5.0/editors/protobuf-mode.el
@@ -66,6 +66,10 @@
 (require 'cc-mode)

 (eval-when-compile
+  (and (= emacs-major-version 24)
+       (= emacs-minor-version 4)
+       (null emacs-repository-version)
+       (require 'cl))
   (require 'cc-langs)
   (require 'cc-fonts))