Changed Core.cpp so that it should build on both windows and linux

This change allows gtorrent-core to be built on windows. Otherwise, the compiler throws an error: 

```
C:/msys32/home/Nick/gtorrent-ncurses/lib/gtorrent-core/src/Core.cpp:50:21: error: no match for 'operator=' (operand types are 'std::vector<char>' and 'std::vector<char>*')
  params.resume_data = resumedata; //TODO: Look if fast resume data exists for this torrent
```

NOTE: I have not tested this in any gui-kit, so I don't know whether this will affect functionality.