[PWGCF] Fix ClassDef compilation warning

[vkucera]

No description provided.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 129 warnings, 🔕 0 disabled

[vkucera]

@victor-gonzalez , there are also these warnings:

Warning: Class or struct o2::analysis::PWGCF::EventSelectionFilterAndAnalysis::ComplexBrickHelper was selected but its dictionary cannot be generated: this is a private or protected class and this is not supported. No direct I/O operation of o2::analysis::PWGCF::EventSelectionFilterAndAnalysis::ComplexBrickHelper instances will be possible.
Warning: Class or struct o2::analysis::PWGCF::EventSelectionFilterAndAnalysis::MultiplicityBrick was selected but its dictionary cannot be generated: this is a private or protected class and this is not supported. No direct I/O operation of o2::analysis::PWGCF::EventSelectionFilterAndAnalysis::MultiplicityBrick instances will be possible.
Warning: Class or struct o2::analysis::PWGCF::EventSelectionFilterAndAnalysis::PileUpRejBrick was selected but its dictionary cannot be generated: this is a private or protected class and this is not supported. No direct I/O operation of o2::analysis::PWGCF::EventSelectionFilterAndAnalysis::PileUpRejBrick instances will be possible.

Can you please try to see if they can be fixed too?

[alibuild]

Error while checking build/O2Physics/staging for 677f035 at 2026-09-28 16:20:

No log files found

Full log here.

[victor-gonzalez]

@vkucera Thanks!!!