[CMake] Fixed include order of xdmfdiff-tool

Lars Bilkerequested to merge
bilke/ogs:xdmfdiff-include into master

The tool is now included before the benchmark tests are processed. This should fix an issue reported by @TobiasMeisel where a test which uses xdmfdiff could not be added to CMake.

  1. Feature description was added to the changelog
Edited by Tobias Meisel

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