[ast2] Incomplete struct when modifying symbol in onSymbol
This took way too long to find! Here's the C code:

```c
struct SDL_AudioCVT;

typedef struct SDL_AudioCVT
{
    int needed;
}  SDL_AudioCVT;
```

and if I have a wrapper written like so:

```nim
import nimterop / [build, cimport]

static:
  cDebug()
  cDisableCaching()

cPlugin:
  import strutils

# Symbol renaming examples
  proc onSymbol*(sym: var Symbol) {.exportc, dynlib.} =
    # Remove prefixes or suffixes from procs
    if sym.name.startsWith("SDL_"):
      sym.name = sym.name.replace("SDL_", "")

cImport("header.h", recurse=true, flags = "-f=ast2")
```

Then the compile output will be:

```nim
# Importing /Users/joey/Downloads/scratch/testover.h
# Generated at 2020-04-12T15:57:58-06:00
# Command line:
#   /Users/joey/.nimble/pkgs/nimterop-#cpluginpath/nimterop/toast --preprocess --recurse -f=ast2 --pnim --nim:/Users/joey/Nim/bin/nim --pluginSourcePath=/Users/joey/.cache/nim/nimterop/cPlugins/nimterop_723324122.nim /Users/joey/Downloads/scratch/testover.h

{.hint[ConvFromXtoItselfNotNeeded]: off.}

import nimterop/types

# type 'SDL_AudioCVT' is duplicate, skipped
# type 'SDL_AudioCVT' is duplicate, skipped

{.pragma: imptestover, importc.}
{.pragma: imptestoverC, imptestover, cdecl.}
type
  AudioCVT* {.incompleteStruct.} = object
```

If I delete the `onSymbol` definition, or I modify the code to not touch the symbol's name, I get the expected output: 

```nim
# Importing /Users/joey/Downloads/scratch/testover.h
# Generated at 2020-04-12T16:00:59-06:00
# Command line:
#   /Users/joey/.nimble/pkgs/nimterop-#cpluginpath/nimterop/toast --preprocess --recurse -f=ast2 --pnim --nim:/Users/joey/Nim/bin/nim /Users/joey/Downloads/scratch/testover.h

{.hint[ConvFromXtoItselfNotNeeded]: off.}

import nimterop/types

# type 'SDL_AudioCVT' is duplicate, skipped
# type 'SDL_AudioCVT' is duplicate, skipped

{.pragma: imptestover, importc.}
{.pragma: imptestoverC, imptestover, cdecl.}
type
  SDL_AudioCVT* {.bycopy.} = object
    needed*: cint
```