Updating Meson build for building External encryption code

[argmarco-tkd]

Bring Meson build files up-to-date for building External encryption code

Testing

• Existing tests pass locally (via ctest -L parquet)
• Github Actions execute correctly
  • https://github.com/protegrity/arrow/actions/runs/22235318775/job/64325334036

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

Opening GitHub issues ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename the pull request title in the following format?

GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[sofia-tekdatum]

Thank you!!