Fix loop variable privatization

[jblueh]

Proposed Changes

mini-PR that adds missing privatization to a loop variable

PR Checklist

• [X] I am submitting my contribution to the develop branch.
• [x] My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• [X] My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• [X] I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• [X] I have added a test case that demonstrates my contribution, if necessary.
• [X] I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.