[ast2] Named struct is missing?
This is a strange one. Given this C code on ast2:

```c
typedef struct GPU_Renderer GPU_Renderer;
typedef struct GPU_Target GPU_Target;

struct GPU_Image {
    int stuff;
} GPU_Image;

struct GPU_Renderer
{
	GPU_Target* current_context_target;
    struct GPU_Image* image;
};

struct GPU_Target
{
	struct GPU_Renderer* renderer;
	GPU_Target* context_target;
};
```

GPU_Image is completely missing from the output:

```nim
# Importing /Users/joey/Downloads/scratch/testover.h
# Generated at 2020-04-12T08:30:33-06:00
# Command line:
#   /Users/joey/.nimble/pkgs/nimterop-#head/nimterop/toast --preprocess -f=ast2 --pnim --nim:/Users/joey/Nim/bin/nim /Users/joey/Downloads/scratch/testover.h

{.hint[ConvFromXtoItselfNotNeeded]: off.}

import nimterop/types

# type 'GPU_Renderer' is duplicate, skipped
# type 'GPU_Target' is duplicate, skipped

{.pragma: imptestover, importc.}
{.pragma: imptestoverC, imptestover, cdecl.}
type
  GPU_Renderer* {.bycopy.} = object
    current_context_target*: ptr GPU_Target
    image*: ptr GPU_Image

  GPU_Target* {.bycopy.} = object
    renderer*: ptr GPU_Renderer
    context_target*: ptr GPU_Target
```

But if I change the GPU_Image struct to:

```c
struct GPU_Image {
    int stuff;
};
```

The output is as expected. This is `ast2` on Nim devel and latest nimterop.