Use flat_stream in ssl_stream example code

example/advanced/server-flex/CMakeLists.txt

@@ -15,6 +15,7 @@ if (OPENSSL_FOUND)
     add_executable (advanced-server-flex
         ${BOOST_BEAST_FILES}
         ${PROJECT_SOURCE_DIR}/example/common/detect_ssl.hpp
+        ${PROJECT_SOURCE_DIR}/example/common/flat_stream.hpp
         ${PROJECT_SOURCE_DIR}/example/common/server_certificate.hpp
         ${PROJECT_SOURCE_DIR}/example/common/ssl_stream.hpp
         Jamfile