Minor fix for Windows build with zlib (facebook#9699)

Summary:
```
conversion from 'size_t' to 'uLong', possible loss of data
```

Fix facebook#9688

Pull Request resolved: facebook#9699

Reviewed By: riversand963

Differential Revision: D34901116

Pulled By: jay-zhuang

fbshipit-source-id: 969148a7a8c023449bd85055a1f0eec71d0a9b3f

util/compression.h

@@ -779,7 +779,8 @@ inline bool Zlib_Compress(const CompressionInfo& info,
   }
 
   // Get an upper bound on the compressed size.
-  size_t upper_bound = deflateBound(&_stream, length);
+  size_t upper_bound =
+      deflateBound(&_stream, static_cast<unsigned long>(length));
   output->resize(output_header_len + upper_bound);
 
   // Compress the input, and put compressed data in output.