Fix `call_snprintf` on aarch64

[nazar-pc]

Essentially c_char can be both signed and unsigned depending on the platform.

Fixes https://github.com/HadrienG2/hwlocality/issues/85

[semanticdiff-com[bot]]

Review changes with SemanticDiff.

Analyzed 1 of 1 files.

	Filename	Status
:heavy_check_mark:	src/ffi/mod.rs	Analyzed

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (3141847) 65.81% compared to head (b57ac2d) 65.81%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #86 +/- ## ======================================= Coverage 65.81% 65.81% ======================================= Files 43 43 Lines 4599 4599 ======================================= Hits 3027 3027 Misses 1572 1572 ``` | [Flag](https://app.codecov.io/gh/HadrienG2/hwlocality/pull/86/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hadrien+G.) | Coverage Δ | | |---|---|---| | [ubuntu-latest](https://app.codecov.io/gh/HadrienG2/hwlocality/pull/86/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hadrien+G.) | `66.97% <100.00%> (-0.10%)` | :arrow_down: | | [windows-latest](https://app.codecov.io/gh/HadrienG2/hwlocality/pull/86/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hadrien+G.) | `64.99% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Hadrien+G.#carryforward-flags-in-the-pull-request-comment) to find out more.

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