also test for charconv library

[sdebionne]

Adding charconv to the list of build libraries.

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[h-vetinari]

The title is a bit misleading. The only place where we're "adding" charconv is in testing that the required (shared) libraries are there at the end of the build.

This can be seen also since the 1.85 from #196 were failing specifically in charconv already before this PR (and the patch to fix it). Long story short, this is a reasonable test improvement, but not worth a separate build (so we can just leave the build number as-is).