g_memdup is deprecated

This warning was raised when compiling the MixStream package:

```
mixstream/MixStream.c: In function ‘mix_stream_play’:
mixstream/MixStream.c:355:3: warning: ‘g_memdup’ is deprecated. Use 'g_memdup2' instead [-Wdeprecated-declarations]
```

`g_memdup` is deprecated since GLib 2.68.