NetBSD Build Errors

Got someone else online to test boost process for me on their NetBSD machine and these are the errors they got:

```
In file included from ../../../../libs/filesystem/src/directory.cpp:38:
../../../../libs/filesystem/src/directory.cpp: In function ‘int boost::filesystem::detail::{anonymous}::dir_itr_fd(const boost::filesystem::detail::dir_itr_imp&, boost::system::error_code&)’:
../../../../libs/filesystem/src/directory.cpp:273:16: error: expected id-expression before ‘(’ token
273 | int fd = ::dirfd(static_cast< DIR* >(imp.handle));
| ^~~~~
```
```
In file included from ../../src/posix/close_handles.cpp:88:
../../src/posix/close_handles.cpp: In function ‘void boost::process::v2::posix::detail::close_all(const std::vector&, boost::system::error_code&)’:
../../src/posix/close_handles.cpp:188:21: error: expected id-expression before ‘(’ token
188 | auto dir_fd = ::dirfd(dir.get());
| ^~~~~
```

Note, the above errors should be all that's left hopefully once [my recent pull request](https://github.com/boostorg/process/pull/415) gets merged.

Special thanks to @NishiOwO for testing boost process on NetBSD using my pull request's branch.