LibWeb: Fix some GCVerifier warnings

Author: https://github.com/mattco98
Commit: 8450041b52
Pull-request: https://github.com/SerenityOS/serenity/pull/23861
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -714,7 +714,7 @@ bool fast_matches(CSS::Selector const& selector, Optional<CSS::CSSStyleSheet con
     // NOTE: If we fail after following a child combinator, we may need to backtrack
     //       to the last matched descendant. We store the state here.
     struct {
-        DOM::Element const* element = nullptr;
+        JS::GCPtr<DOM::Element const> element;
         ssize_t compound_selector_index = 0;
     } backtrack_state;