Fix another sprintf warning in example.

asio/src/examples/cpp11/chat/chat_message.hpp

diff --git a/asio/src/examples/cpp11/chat/chat_message.hpp b/asio/src/examples/cpp11/chat/chat_message.hpp
index 770a7d3..3751504 100644
--- a/asio/src/examples/cpp11/chat/chat_message.hpp
+++ b/asio/src/examples/cpp11/chat/chat_message.hpp
@@ -79,7 +79,7 @@
   void encode_header()
   {
     char header[header_length + 1] = "";
-    std::sprintf(header, "%4d", body_length_);
+    std::sprintf(header, "%4d", static_cast<int>(body_length_));
     std::memcpy(data_, header, header_length);
   }