Allow dead code for an error enum that is not being used

We do generate css::ParseErrorKind at some point, I think, but we
don't consult it or print it.  No idea where to do that, i.e. at what
point after calling some API from cssparser or selectors.

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/985>

Author: federicomenaquintero

rsvg/src/css.rs

@@ -186,6 +186,7 @@ pub struct RuleParser {
 }
 
 /// Errors from the CSS parsing process
+#[allow(dead_code)] // looks like we are not actually using this yet?
 #[derive(Debug)]
 pub enum ParseErrorKind<'i> {
     Selector(selectors::parser::SelectorParseErrorKind<'i>),