Fix variable-length-array warning clang18

[BeyondEspresso]

The PR fixes a warning triggered by variable length arrays in Clang 18:

In file included from /ccs/home/adebus/src/picongpu/include/picongpu/plugins/openPMD/openPMDWriter.x.cpp:50:
/ccs/home/adebus/src/picongpu/include/picongpu/../picongpu/plugins/openPMD/WriteSpecies.hpp:429:40: warning: variable length arrays in C++ are a Clang extension [-Wvla-cxx-extension]
  429 |                     uint64_t numRounds[mpiSize];
      |                                        ^~~~~~~
/ccs/home/adebus/src/picongpu/include/picongpu/../picongpu/plugins/openPMD/WriteSpecies.hpp:429:40: note: read of non-const variable 'mpiSize' is not allowed in a constant expression
/ccs/home/adebus/src/picongpu/include/picongpu/../picongpu/plugins/openPMD/WriteSpecies.hpp:342:26: note: declared here
  342 |                 uint64_t mpiSize = gc.getGlobalSize();
      |                          ^