Fix the warning about redundant import of impl_buffered_source when async-tokio feature is not enabled

Fixes the warning:
```
warning: the item `impl_buffered_source` is imported redundantly
   --> src\reader\buffered_reader.rs:235:16
    |
13  | / macro_rules! impl_buffered_source {
14  | |     ($($lf:lifetime, $reader:tt, $async:ident, $await:ident)?) => {
15  | |         #[cfg(not(feature = "encoding"))]
16  | |         $($async)? fn remove_utf8_bom(&mut self) -> Result<()> {
...   |
231 | |     };
232 | | }
    | |_- the item `impl_buffered_source` is already defined here
...
235 |   pub(super) use impl_buffered_source;
    |                  ^^^^^^^^^^^^^^^^^^^^
```

Author: Mingun

src/reader/buffered_reader.rs

@@ -231,7 +231,11 @@ macro_rules! impl_buffered_source {
     };
 }
 
-// Make it public for use in async implementations
+// Make it public for use in async implementations.
+// New rustc reports
+// > warning: the item `impl_buffered_source` is imported redundantly
+// so make it public only when async feature is enabled
+#[cfg(feature = "async-tokio")]
 pub(super) use impl_buffered_source;
 
 /// Implementation of `XmlSource` for any `BufRead` reader using a user-given