Fix: abstract EventLoop::Polling#system_add invalid signature

The abstract method refers to the non existing `Index` type. Weirdly the compiler won't complain until someone defines an `Index` type.

Resolves #15357

src/crystal/event_loop/polling.cr

@@ -515,7 +515,7 @@ abstract class Crystal::EventLoop::Polling < Crystal::EventLoop
   private abstract def system_run(blocking : Bool, & : Fiber ->) : Nil
 
   # Add *fd* to the polling system, setting *index* as user data.
-  protected abstract def system_add(fd : Int32, index : Index) : Nil
+  protected abstract def system_add(fd : Int32, index : Arena::Index) : Nil
 
   # Remove *fd* from the polling system. Must raise a `RuntimeError` on error.
   #