[PWGLF,PWGMM] fix flattenicity: filling histograms error

[PaolaVT]

No description provided.

[github-actions]

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

[alibuild]

Error while checking build/O2Physics/code-check for e7a3043 at 2026-08-22 21:37:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 3 errors and 2 warnings.'
Found 3 errors and 2 warnings.
++ [[ 3 -gt 0 ]]
++ cat /sw/BUILD/4a9f6a5047e316e8d55ff2c673b79356d09f6c2c/O2Physics-code-check/errors.txt
PWGMM/UE/Tasks/flattenicityTask.cxx:18:1: error: included header Multiplicity.h is not used directly [misc-include-cleaner,-warnings-as-errors]
PWGMM/UE/Tasks/flattenicityTask.cxx:510:17: error: no header providing "std::size_t" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGMM/UE/Tasks/flattenicityTask.cxx:511:9: error: no header providing "uint8_t" is directly included [misc-include-cleaner,-warnings-as-errors]
++ [[ 2 -gt 0 ]]
++ cat /sw/BUILD/4a9f6a5047e316e8d55ff2c673b79356d09f6c2c/O2Physics-code-check/warnings.txt
PWGMM/UE/Tasks/flattenicityTask.cxx:509:9: warning: the 'empty' method should be used to check for emptiness instead of 'size' [readability-container-size-empty]
PWGMM/UE/Tasks/flattenicityTask.cxx:519:9: warning: the 'empty' method should be used to check for emptiness instead of 'size' [readability-container-size-empty]
++ [[ 3 -gt 0 ]]
++ exit 1
--

Full log here.