AK: Fix IntrusvieList::take_first/last() actually compile with RefPtr<T>

PR SerenityOS#6376 made IntrusiveList capable of holding RefPtr<T>, etc. however
there was a latent bug where take_first() / take_last() would fail to
compile because they weren't being converted to their container type.

AK/IntrusiveList.h

@@ -246,7 +246,7 @@ inline Container IntrusiveList<T, Container, member>::first() const
 template<class T, typename Container, IntrusiveListNode<T, Container> T::*member>
 inline Container IntrusiveList<T, Container, member>::take_first()
 {
- if (auto* ptr = first()) {
+ if (Container ptr = first()) {
  remove(*ptr);
  return ptr;
  }
@@ -256,7 +256,7 @@ inline Container IntrusiveList<T, Container, member>::take_first()
 template<class T, typename Container, IntrusiveListNode<T, Container> T::*member>
 inline Container IntrusiveList<T, Container, member>::take_last()
 {
- if (auto* ptr = last()) {
+ if (Container ptr = last()) {
  remove(*ptr);
  return ptr;
  }