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/DOM/MutationObserver.cpp

@@ -12,6 +12,7 @@
 namespace Web::DOM {
 
 JS_DEFINE_ALLOCATOR(MutationObserver);
+JS_DEFINE_ALLOCATOR(TransientRegisteredObserver);
 
 WebIDL::ExceptionOr<JS::NonnullGCPtr<MutationObserver>> MutationObserver::construct_impl(JS::Realm& realm, JS::GCPtr<WebIDL::CallbackType> callback)
 {