Fix `get_renf` deprecation warnings

```
deps/src/normaliz.cpp:62:37: warning: ‘renf* eantic::renf_class::get_renf() const’ is deprecated: Use renf_t() instead. [-Wdeprecated-declarations]
```
Unfortunately the recommendation given there (replacing `x.get_renf()` by `renf_t(x)`) does not work.