Fixes a warning reported on Discord.

When I install libjxl-0.11.1 on Gentoo, following message is displayed:

• QA Notice: Compatibility w/ CMake < 3.16 will be removed in future ECM release.
• If not fixed in upstream's code repository, we should make sure they are aware.
• See also tracker bug #964407; check existing or file a new bug for this package.
• The following CMakeLists.txt files are causing warnings:
• examples/CMakeLists.txt:3.10
• An upstreamable patch should take any resulting CMake policy changes
• into account. See also:
• https://cmake.org/cmake/help/latest/manual/cmake-policies.7.html