[PWGLF] Use std::abs for charge comparison

[njacazio]

No description provided.

[github-actions]

O2 linter results: ❌ 3 errors, ⚠️ 2 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/code-check for f056bb9 at 2026-08-18 10:22:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 5 errors and 2 warnings.'
Found 5 errors and 2 warnings.
++ [[ 5 -gt 0 ]]
++ cat /sw/BUILD/4a425fc86b7585c4a4e40c8f6d903ced71ee7bcb/O2Physics-code-check/errors.txt
PWGLF/Utils/inelGt.h:29:1: error: nested namespaces can be concatenated [modernize-concat-nested-namespaces,-warnings-as-errors]
PWGLF/Utils/inelGt.h:44:3: error: uninitialized record type: 'etaCharge' [cppcoreguidelines-pro-type-member-init,-warnings-as-errors]
PWGLF/Utils/inelGt.h:51:17: error: use designated initializer list to initialize 'EtaCharge' [modernize-use-designated-initializers,-warnings-as-errors]
PWGLF/Utils/inelGt.h:77:12: error: redundant boolean literal in conditional return statement [readability-simplify-boolean-expr,-warnings-as-errors]
PWGLF/Utils/inelGt.h:78:5: error: do not use 'else' after 'return' [readability-else-after-return,-warnings-as-errors]
++ [[ 2 -gt 0 ]]
++ cat /sw/BUILD/4a425fc86b7585c4a4e40c8f6d903ced71ee7bcb/O2Physics-code-check/warnings.txt
PWGLF/Utils/inelGt.h:145:42: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/Utils/inelGt.h:148:43: warning: statement should be inside braces [readability-braces-around-statements]
++ [[ 5 -gt 0 ]]
++ exit 1
--

Full log here.