Core: fix missing import for `MultiWorld.link_items()` (#3731)

BaseClasses.py

@@ -290,6 +290,8 @@ class MultiWorld():
 
     def link_items(self) -> None:
         """Called to link together items in the itempool related to the registered item link groups."""
+        from worlds import AutoWorld
+
         for group_id, group in self.groups.items():
             def find_common_pool(players: Set[int], shared_pool: Set[str]) -> Tuple[
                 Optional[Dict[int, Dict[str, int]]], Optional[Dict[str, int]]