Fix for Mantis 6221.

Fix for [Mantis 6221](http://caml.inria.fr/mantis/view.php?id=6221).

Test case:

```
$ cat field.ml
<:ctyp< a:b >>
```

Before:

```
$ camlp4of field.ml
File "field.ml", line 1, characters 10-11:
While expanding quotation "ctyp" in a position of "str_item":
  Parse error: "->" expected after [ctyp level star] (in [ctyp])
```

After:

```
$ camlp4of field.ml
Ast.TyCol (_loc, (Ast.TyId (_loc, (Ast.IdLid (_loc, "a")))),
  (Ast.TyId (_loc, (Ast.IdLid (_loc, "b")))))
```