fix compile by specifying void type for add_size

Fixes this error:

```cpp
In file included from ../include/osmium/ser/item.hpp:36:
../include/osmium/ser/buffer.hpp:165:21: error: C++ requires a type specifier for all declarations
            virtual add_size(length_t size) {
            ~~~~~~~ ^
```

And here are warnings fyi: https://gist.github.com/springmeyer/d73d50a51ddfc96c9cb7