343 missed updating the CMake build config. (Sadly this project has no working CI.)

Weirdly, the missing Sendable conformances are only causing problems on Swift 5.5 -- they must be inferred elsewhere.

[X] I've read the Contribution Guidelines
[X] My contributions are licensed under the Swift license.
[X] I've followed the coding style of the rest of the project.
[ ] I've added tests covering all new code paths my change adds to the project (if appropriate).
[ ] I've added benchmarks covering new functionality (if appropriate).
[X] I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
[ ] I've updated the documentation if necessary.

apple / swift-collections