fix test import

tests/serde-issues.rs

@@ -8,6 +8,7 @@ use quick_xml::se::{to_string, to_string_with_root};
 use serde::de::{Deserializer, IgnoredAny};
 use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
+use std::io::BufReader;
 
 /// Regression tests for https://github.com/tafia/quick-xml/issues/252.
 mod issue252 {