Fix for compilation failure ESS/MCPL instrument, openacc

[willend]

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Please describe what your PR is adding in terms of features or bugfixes:

Divisons must be USERVAR to function with openacc
(fix for compilation failure, https://new-nightly.mcstas.org/todays-datafiles/McStas_8GPU_openacc_mpi_x_8_1e7_Linux_20260813_0001_07_1e7/ESS_butterfly_MCPL_test/compile_stdout.txt)

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Declaration of use of AI-tools

• Please add a checkmark here if you used AI-tools during the work for this contribution
• Furter, please describe how / where and for what the tools were used:

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Development OS / boundary conditions

Please describe what OS you developed and tested your additions on, and if any special dependencies are required:

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PR Checklist for contributing to McStas/McXtrace

For a coherent and useful contribution to McStas/McXtrace, please fill in relevant parts of the checklist:

• My contribution includes patches to an existing instrument file

  • I have used the mcdoc utility and rendered a reasonable documentation page for the instrument (please attach as screenshot in comments!)
  • I have used the mctest utility to test the instrument (please attach mcviewtest report as screenshot in comments)
  • I have used the mcrun --c-lint "linter" and followed advice to remove most / all warnings that are raised

• My contribution contains something else

  • Explanation is added in free form text above or below the checklist

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