Latest nightly compiler reports dead_code warnings for structs with private fields

[asomers]

Sometime in the last week the rustc nightly compiler got more aggressive about reporting dead_code warnings. It now warns whenever a struct contains a private field but no constructor. That catches many structs in libc, which are intended to be initialized with std::mem::zeroed(). IMHO this is a rustc bug. This ought to be a separate lint so it can be silenced without silencing all dead_code warnings.

Here's an example build failure: https://cirrus-ci.com/task/6392171014651904?logs=test#L17