👨‍🌾 Gci and Rci report cmake lint errors

[jacobperron]

Both the Galactic and Rolling performance jobs are reporting cmake lint errors for src/idlgen/opendds/CMakeLists.txt, which I presume is a third-party package. Probably we should not be running linters on that package.

Example failures:

• https://build.ros2.org/view/Gci/job/Gci__nightly-performance_ubuntu_focal_amd64/24/testReport/junit/(root)/projectroot/lint_cmake/
• https://build.ros2.org/view/Rci/job/Rci__nightly-performance_ubuntu_focal_amd64/351/testReport/

[clalancette]

Fixed by #38, so closing.