Fix Windows build. %zu isn't supported in the crt implementation

we use there, so it just crashes.
Review URL: http://webrtc-codereview.appspot.com/213001

git-svn-id: http://webrtc.googlecode.com/svn/trunk@712 4adac7df-926f-26a2-2b94-8c16560cd09d

peerconnection/samples/server/utils.h

@@ -18,5 +18,6 @@
 #endif
 
 std::string int2str(int i);
+std::string size_t2str(size_t i);
 
 #endif  // PEERCONNECTION_SAMPLES_SERVER_UTILS_H_