Build fails for Android targets

In `dlopen2/src/raw/unix.rs`:

```rust
#[cfg(not(any(target_os = "linux", target_os = "macos")))]
use {
    once_cell::sync::Lazy,
    std::sync::{LockResult, Mutex, MutexGuard},
};
```

However, `once_cell` is not added in `Cargo.toml` as a dependency for non-Linux, non-MacOS Unix targets.