va_list for callbacks

```c
void fn(void (*callback)(const char*, va_list));
```

was still generated as
```ruby
attach_function :fn, :fn, [ callback([ :string, va_list ], :void) ], :void
```

this PR fixes it, but not sure if this is best way to implement it.
Merge pull request #34 from davispuh/patch-1

va_list for callbacks