Exported default function types not correct

This module exports the default as not a function, which results on projects trying to compile typescript using this module as 

```
Type 'typeof import("/home/trout/tmp/js-libp2p/node_modules/private-ip/lib/index")' has no call signatures.
```

Ref https://github.com/libp2p/js-libp2p/issues/1019