xso: fix no_std support

This reverts the Revert in 6ec275d3 which had to be done in order to
un-break usage of xso in std crates.

xso-proc/src/structs.rs

@@ -294,7 +294,7 @@ impl StructInner {
                     quote! {
                         let name = (
                             ::xso::exports::rxml::Namespace::from(#xml_namespace),
-                            ::std::borrow::Cow::Borrowed(#xml_name),
+                            ::xso::exports::alloc::borrow::Cow::Borrowed(#xml_name),
                         );
                         #init
                     }