LibWeb: Add calls to JS_{DECLARE,DEFINE}_ALLOCATOR()

Author: https://github.com/mattco98
Commit: 31341b280a
Pull-request: https://github.com/SerenityOS/serenity/pull/23878
Issue: https://github.com/SerenityOS/serenity/issues/23805
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/HTML/Scripting/Fetching.h

@@ -56,6 +56,7 @@ ScriptFetchOptions default_classic_script_fetch_options();
 
 class FetchContext : public JS::GraphLoadingState::HostDefined {
     JS_CELL(FetchContext, JS::GraphLoadingState::HostDefined);
+    JS_DECLARE_ALLOCATOR(FetchContext);
 
 public:
     JS::Value parse_error;                                    // [[ParseError]]