pkgs-lib/hocon: fix type for `_includes`

pkgs/pkgs-lib/formats/hocon/default.nix

···
                 path
                 str
               ]);
+
+              includeType = addCheck attrs (x: (x._type or null) == "include");
             in
             (oneOf [
               atomType
-              (listOf atomType)
+              (addCheck (listOf atomType) (lib.all atomType.check))
+              (addCheck (listOf includeType) (lib.all includeType.check))
               (attrsOf type')
             ])
             // {