ctypes: missing rule for stubs when only using a type description

Hi,

When a ctypes description has just a type description, Dune does not setup a rule for a stubs file it tries to use.

```
(library
 (name l)
 (ctypes
  (external_library_name none)
  (build_flags_resolver vendored)
  (type_description
   (functor bindings)
   (instance types))
  (generated_entry_point c)))
```

```
  Error: No rule found for libl_stubs.a
```