Fixes "no member named 'ca_certs_filename' in 'sp_session_config'"

```
../src/session.cc:310:10: error: no member named 'ca_certs_filename' in 'sp_session_config'
    ptr->ca_certs_filename              = NSP_STRING_KEY(obj, "ca_certs_filename");
    ~~~  ^
1 error generated.
make: *** [Release/obj.target/libspotify/src/session.o] Error 1
```