Fix debug build in 2.3.x

Fixes

```
include/mapnik/png_io.hpp: In function ‘void mapnik::save_as_png8_hex(T1&, const T2&, const mapnik::png_options&)’:
include/mapnik/png_io.hpp:687: error: ‘colors’ was not declared in this scope
```

...which only occurs when configured with `DEBUG=true`.