[FEA] Add extern "C" to generated header, so it can be used from C++ programs

In order to call C libraries from C++, function definitions need to be warped by:

```
#ifdef __cplusplus
extern "C"{
#endif

// Funtion declaration...

#ifdef __cplusplus
}
#endif
```

Automatically adding those lines to the generated header would simplify using classifiers from C++ code