Add buffer_size

include/boost/beast/http/string_body.hpp

@@ -11,6 +11,7 @@
 #define BOOST_BEAST_HTTP_STRING_BODY_HPP
 
 #include <boost/beast/core/detail/config.hpp>
+#include <boost/beast/core/buffer_size.hpp>
 #include <boost/beast/http/error.hpp>
 #include <boost/beast/http/message.hpp>
 #include <boost/beast/core/buffers_range.hpp>
@@ -117,7 +118,6 @@ public:
         put(ConstBufferSequence const& buffers,
             error_code& ec)
         {
-            using net::buffer_size;
             auto const extra = buffer_size(buffers);
             auto const size = body_.size();
             try