fix compiler warning
fix compiler warning

```include/mapnik/text/symbolizer_helpers.hpp:164:45: warning: template template parameter using 'typename' is a C++17 extension
      [-Wc++17-extensions]
    template <template <typename, typename> typename GridAdapter>
                                            ^~~~~~~~
                                            class
```

tomhughes/mapnik,naturalatlas/mapnik,mapnik/mapnik,naturalatlas/mapnik,mapnik/mapnik,tomhughes/mapnik,lightmare/mapnik,tomhughes/mapnik,mapnik/mapnik,lightmare/mapnik,tomhughes/mapnik,naturalatlas/mapnik,lightmare/mapnik,naturalatlas/mapnik,lightmare/mapnik,mapnik/mapnik