[PWGLF] Fix case for neutron in histogram filling

[njacazio]

No description provided.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 1 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/code-check for a4095b2 at 2026-09-20 12:57:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 1 errors and 3 warnings.'
Found 1 errors and 3 warnings.
++ [[ 1 -gt 0 ]]
++ cat /sw/BUILD/70e8132507003ce2c1eb4732adaed4d913e5a23f/O2Physics-code-check/errors.txt
PWGLF/Tasks/Nuspex/hadronnucleicorrelation.cxx:2011:12: error: no header providing "size_t" is directly included [misc-include-cleaner,-warnings-as-errors]
++ [[ 3 -gt 0 ]]
++ cat /sw/BUILD/70e8132507003ce2c1eb4732adaed4d913e5a23f/O2Physics-code-check/warnings.txt
PWGLF/Tasks/Nuspex/hadronnucleicorrelation.cxx:188:5: warning: function 'pt' should be marked [[nodiscard]] [modernize-use-nodiscard]
PWGLF/Tasks/Nuspex/hadronnucleicorrelation.cxx:189:5: warning: function 'eta' should be marked [[nodiscard]] [modernize-use-nodiscard]
PWGLF/Tasks/Nuspex/hadronnucleicorrelation.cxx:190:5: warning: function 'phi' should be marked [[nodiscard]] [modernize-use-nodiscard]
++ [[ 1 -gt 0 ]]
++ exit 1
--

Full log here.

[vkucera]

@njacazio Please fix the error.