rnp fails to build on GNU/Hurd (`MAXPATHLEN` is not available)

This is similar to #1665:

```
/build/reproducible-path/rnp-0.18.0/src/librekey/rnp_key_store.cpp: In member function ‘bool rnp::KeyStore::write()’:
/build/reproducible-path/rnp-0.18.0/src/librekey/rnp_key_store.cpp:129:21: error: ‘MAXPATHLEN’ was not declared in this scope; did you mean ‘MAXNAMLEN’?
  129 |         char chpath[MAXPATHLEN];
      |                     ^~~~~~~~~~
```