fix build on 32-bits, cf https://github.com/mapnik/mapnik/issues/4517

https://github.com/mapnik/mapnik/commit/210c463230194a04104cce2126f558cb5bcdeb5c
Index: plugins/input/tiles/pmtiles_source.hpp
--- plugins/input/tiles/pmtiles_source.hpp.orig
+++ plugins/input/tiles/pmtiles_source.hpp
@@ -522,7 +522,7 @@ class pmtiles_source : public tiles_source
         }
         else
         {
-            metadata = {data() + metadata_offset, metadata_length};
+            metadata = {data() + metadata_offset, static_cast<std::size_t>(metadata_length)};
         }
         boost::json::value json_value;
         try
