[PWGLF] Add flattenicity response

[gbencedi]

No description provided.

[github-actions]

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

[alibuild]

Error while checking build/O2Physics/code-check for 0e33aee at 2026-07-28 13:33:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 4 errors and 0 warnings.'
Found 4 errors and 0 warnings.
++ [[ 4 -gt 0 ]]
++ cat /sw/BUILD/e3440e6892d119c91f67857206b146470f5a49e3/O2Physics-code-check/errors.txt
PWGLF/Tasks/GlobalEventProperties/flattenictyPikp.cxx:940:25: error: no header providing "std::is_pointer" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGLF/Tasks/GlobalEventProperties/flattenictyPikp.cxx:1881:45: error: repeated branch body in conditional chain [bugprone-branch-clone,-warnings-as-errors]
PWGLF/Tasks/GlobalEventProperties/flattenictyPikp.cxx:1887:45: error: repeated branch body in conditional chain [bugprone-branch-clone,-warnings-as-errors]
PWGLF/Tasks/GlobalEventProperties/flattenictyPikp.cxx:1893:46: error: repeated branch body in conditional chain [bugprone-branch-clone,-warnings-as-errors]
++ [[ 0 -gt 0 ]]
++ [[ 4 -gt 0 ]]
++ exit 1
--

Full log here.