[PWGDQ] Clang-tidy fixes in HistogramManager and HistogramsLibrary

[mcoquet642]

No description provided.

[github-actions]

O2 linter results: ❌ 54 errors, ⚠️ 143 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/code-check for 62fdeef at 2026-07-13 18:39:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 5 errors and 0 warnings.'
Found 5 errors and 0 warnings.
++ [[ 5 -gt 0 ]]
++ cat /sw/BUILD/ef28b65b61cc6c849b72bda016a712eb2f458fd9/O2Physics-code-check/errors.txt
PWGDQ/Core/HistogramManager.cxx:97:54: error: no header providing "std::size_t" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGDQ/Core/HistogramManager.h:49:5: error: 'if' statement is unnecessary; deleting null pointer has no effect [readability-delete-null-pointer,-warnings-as-errors]
PWGDQ/Core/HistogramManager.h:94:3: error: function 'GetUsedVars' should be marked [[nodiscard]] [modernize-use-nodiscard,-warnings-as-errors]
PWGDQ/Core/HistogramManager.h:98:3: error: function 'GetAllocatedBins' should be marked [[nodiscard]] [modernize-use-nodiscard,-warnings-as-errors]
PWGDQ/Core/HistogramsLibrary.cxx:31:1: error: included header algorithm is not used directly [misc-include-cleaner,-warnings-as-errors]
++ [[ 0 -gt 0 ]]
++ [[ 5 -gt 0 ]]
++ exit 1
--

Full log here.

[alibuild]

Error while checking build/O2Physics/o2 for 62fdeef at 2026-07-13 19:10:

## sw/BUILD/O2Physics-latest/log
c++: fatal error: Killed signal terminated program cc1plus
ninja: build stopped: subcommand failed.

Full log here.