Fixed a wrong property in CMake

[hillmich]

• Make nlohmann_json a system include to suppress potential warnings (see https://cmake.org/cmake/help/latest/prop_tgt/INTERFACE_SYSTEM_INCLUDE_DIRECTORIES.html for documentation)

[codecov]

Codecov Report

Merging #257 (b7ee319) into main (a7d0ab9) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #257   +/-   ##
=====================================
  Coverage   84.0%   84.0%           
=====================================
  Files         58      58           
  Lines       6974    6974           
  Branches    1387    1387           
=====================================
  Hits        5860    5860           
  Misses      1114    1114           

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[burgholzer]

Thanks for catching that mistake 👍🏼