[PWGEM] Fix code check errors

[jokonig]

• Fix missing includes or remove unnecessary includes
• Add safeguard to still use global using declaration (since its not a real header file, its okay to define using globally)
• partially introduced in [PWGEM] Add Event Skimmer for Dalitz taking both V0 and Elec into acc… #17787

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 145 warnings, 🔕 27 disabled

[alibuild]

Auto-approving on behalf of @jokonig.

[vkucera]

Hi @jokonig , thanks for the adjustment.
I don't understand why you say that "it's not a real header file" though.

[jokonig]

Hi @vkucera
What I meant with that is that the photon-conversion-builder and dalitz-skimmer are in principle tasks. However, since we had to template it to allow the same task to handle different tables as input, we moved the task into a header file to have two different .cxx files to call it with different template arguments.
Of course, it is a header file, but its not a header file that is used by different user tasks etc. and hence we thought it is okay to keep the global namespaces.

[vkucera]

Hi @vkucera What I meant with that is that the photon-conversion-builder and dalitz-skimmer are in principle tasks. However, since we had to template it to allow the same task to handle different tables as input, we moved the task into a header file to have two different .cxx files to call it with different template arguments. Of course, it is a header file, but its not a header file that is used by different user tasks etc. and hence we thought it is okay to keep the global namespaces.

OK, thanks for the clarification.