fix compile error with Rust 1.64

[yu-re-ka]

As of rustc 1.64, Box::from_raw is #[must_use].

error: unused return value of `Box::<T>::from_raw` that must be used
   --> flux/src/cffi.rs:100:5
    |
100 |     Box::from_raw(cstr);
    |     ^^^^^^^^^^^^^^^^^^^^
    |
note: the lint level is defined here
   --> flux/src/lib.rs:1:38
    |
1   | #![cfg_attr(feature = "strict", deny(warnings, missing_docs))]
    |                                      ^^^^^^^^
    = note: `#[deny(unused_must_use)]` implied by `#[deny(warnings)]`
    = note: call `drop(from_raw(ptr))` if you intend to drop the `Box`

error: could not compile `flux` due to previous error

Checklist

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[Marwes]

You need to run make generate and commit the changes, otherwise 👍

[nathanielc]

@yu-re-ka Thanks for the PR have you signed the CLA https://www.influxdata.com/legal/cla/ ?

[yu-re-ka]

Both done

[nathanielc]

@yu-re-ka Thanks again for the contribution!