RFR: Fix curl sample

[Jorn Vernee]

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.

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Commit messages:
 - Fix curl sample

Changes: https://git.openjdk.org/jextract/pull/199/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=199&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/199.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/199/head:pull/199

PR: https://git.openjdk.org/jextract/pull/199