[mini] fix compiler warning for compiling without openPMD

[SeverinDiederichs]

in the diagnostics lev is only used for openPMD, but was declared globally, resulting in an unused parameter warning, if compiled without openPMD. lev is now defined in the openPMD only area, fixing the warning.

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable