xso: fix build with minidom and without std

xso/ChangeLog

@@ -1,6 +1,7 @@
 Version NEXT:
     * Changes
       - Fix some Clippy warnings
+      - Fix build with minidom and without std (!661)
 
 Version 0.3.0, release 2025-10-28:
     * Changes

xso/src/minidom_compat.rs

@@ -9,6 +9,7 @@
 use alloc::{
     borrow::{Cow, ToOwned},
     boxed::Box,
+    string::String,
     vec::IntoIter,
 };
 use core::marker::PhantomData;
@@ -156,7 +157,7 @@ enum AsXmlState<'a> {
         element: &'a Element,
 
         /// Attribute iterator.
-        attributes: rxml::xml_map::Iter<'a, std::string::String>,
+        attributes: rxml::xml_map::Iter<'a, String>,
     },
 
     /// Content: The contents of the element are streamed as events.