[PWGEM/Dilepton] add a flag to use PDG mass of Jpsi

[dsekihat]

[PWGEM/Dilepton] add a flag to use PDG mass of Jpsi

[github-actions]

O2 linter results: ❌ 40 errors, ⚠️ 76 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/code-check for b9d32aa at 2026-08-22 15:13:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 1 errors and 2 warnings.'
Found 1 errors and 2 warnings.
++ [[ 1 -gt 0 ]]
++ cat /sw/BUILD/6bd2ea29cf47f7ee9ebc3da7253ce0f1df917932/O2Physics-code-check/errors.txt
PWGEM/Dilepton/Tasks/dimuonV1.cxx:80:72: error: no header providing "M_PI" is directly included [misc-include-cleaner,-warnings-as-errors]
++ [[ 2 -gt 0 ]]
++ cat /sw/BUILD/6bd2ea29cf47f7ee9ebc3da7253ce0f1df917932/O2Physics-code-check/warnings.txt
PWGEM/Dilepton/Tasks/dimuonV1.cxx:171:20: warning: do not declare C-style arrays, use 'std::array' instead [modernize-avoid-c-arrays]
PWGEM/Dilepton/Tasks/dimuonV1.cxx:200:3: warning: use '= default' to define a trivial destructor [modernize-use-equals-default]
++ [[ 1 -gt 0 ]]
++ exit 1
--

Full log here.

[alibuild]

Auto-approving on behalf of @dsekihat.