RFR: Fix curl sample

[Maurizio Cimadamore]

On Mon, 5 Feb 2024 15:02:32 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> Noticed a small issue in the curl sample. We recently moved the varargs invoker that infers layouts to nested class under the name `invoke`. Switch the sample to the new scheme as well.

Marked as reviewed by mcimadamore (Reviewer).

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PR Review: https://git.openjdk.org/jextract/pull/199#pullrequestreview-1864909134