LibWeb: Fix a few const-ness issues

Author: https://github.com/mattco98
Commit: c0b2fa74ac
Pull-request: https://github.com/SerenityOS/serenity/pull/17620
Issue: https://github.com/SerenityOS/serenity/issues/16988
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibWeb/DOMParsing/XMLSerializer.h

@@ -18,7 +18,7 @@ public:
 
     virtual ~XMLSerializer() override;
 
-    WebIDL::ExceptionOr<DeprecatedString> serialize_to_string(JS::NonnullGCPtr<DOM::Node> root);
+    WebIDL::ExceptionOr<DeprecatedString> serialize_to_string(JS::NonnullGCPtr<DOM::Node const> root);
 
 private:
     explicit XMLSerializer(JS::Realm&);
@@ -31,6 +31,5 @@ enum class RequireWellFormed {
     Yes,
 };
 
-WebIDL::ExceptionOr<DeprecatedString> serialize_node_to_xml_string(JS::NonnullGCPtr<DOM::Node> root, RequireWellFormed require_well_formed);
-
+WebIDL::ExceptionOr<DeprecatedString> serialize_node_to_xml_string(JS::NonnullGCPtr<DOM::Node const> root, RequireWellFormed require_well_formed);
 }