stream_wrap: fix signed/unsigned comparison warning

src/stream_wrap.cc

@@ -576,7 +576,7 @@ int StreamWrapCallbacks::DoWrite(WriteWrap* w,
 
   if (!r) {
     size_t bytes = 0;
-    for (int i = 0; i < count; i++)
+    for (size_t i = 0; i < count; i++)
       bytes += bufs[i].len;
     if (wrap_->stream_->type == UV_TCP) {
       NODE_COUNT_NET_BYTES_SENT(bytes);