Enable run_cppcheck on gyselalibxx repo

[EmilyBourne]

<Describe your PR here. This will form the basis of the squashed commit message in the devel branch.>

---

Please complete the checklist to ensure that all tasks are completed before marking your pull request as ready for review.

All Submissions

• Have you ensured that all lines changed in this PR are justified by a comment found in the description ?
• Have you updated the CHANGELOG.md ?
• Have you linked any issues that should be closed when this PR is merged (using closing keywords) ?
• Have you checked that the AUTHORS file is up to date ?
• Have you checked that the copyright information in the LICENCE file is up to date (including dates) ?
• Do you follow the conventions specified in our coding standards ?

New Feature Submissions

• Have you added tests for the new functionalities ?
• Have you documented the new functionalities:
  • API documentation describing the available methods, when each should be used and how to use them ?
  • User-friendly documentation in README files (which may link to the API documentation).
  • If the new functionality is non-trivial to use, provide a tutorial or example ? (optional)

Changes to Existing Features

• Have you checked that existing tests cover all code after the changes ?
• Have you checked that existing tests are still passing ?
• Have you checked that the existing documentation is still accurate (API and README files) ?

Changes to the CI

• Have you made the same changes to both the GitHub CI and the GitLab CI (for the private fork) ?