`importc` fails to generate stub type
### Description

```nim
type MyType {.importc, incompleteStruct.} = object

var v {.exportc.}: ptr MyType
```



My expectation, since there's no `nodecl`, would be a stub type generated (`typedef struct MyType MyType;` or similar. 


### Nim Version

`devel`, 2.0

### Current Output

```text
nim c testit.nim
/tmp/xxx/@mtest.nim.c:23:15: error: unknown type name ‘MyType’
   23 | N_LIB_PRIVATE MyType* v;
      |               ^~~~~~
```


### Expected Output

_No response_

### Known Workarounds

_No response_

### Additional Information

_No response_