xso: fix build on no_std

xso/ChangeLog

@@ -6,6 +6,7 @@ Version NEXT:
     * Changes
       - Fix some Clippy warnings
       - Fix build with minidom and without std (!661)
+      - Fix no_std build
     * Added
       - xso::from_bytes_with_options and xso::from_reader_with_options to allow
         passing custom parser configuration.

xso/src/lib.rs

@@ -702,6 +702,7 @@ fn read_start_event_io(
     ))
 }
 
+#[cfg(feature = "std")]
 fn from_reader_inner<T: FromXml, R: io::BufRead>(
     mut reader: rxml::XmlLangTracker<rxml::Reader<R>>,
 ) -> io::Result<T> {