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 +66,7 @@
 (require 'cc-mode)
 
 (eval-when-compile
+  (require 'cl)
   (require 'cc-langs)
   (require 'cc-fonts))