don't use deprecated syntax

Co-authored-by: José Valim <[email protected]>

Author: zachdaniel

lib/ecto/query/planner.ex

@@ -2285,7 +2285,7 @@ defmodule Ecto.Query.Planner do
     requested_fields = List.wrap(requested_fields)
 
     Enum.each(requested_fields, fn field ->
-      unless field in available_fields do
+      if field not in available_fields do
         error!(query, "field `#{field}` in struct/2 is not available in the subquery. " <>
                      "Subquery only returns fields: #{inspect(available_fields)}")
       end