quick_tests/affinity: fix warning

[marcfehling]

1/7 Test #2: affinity.release .................***Failed    6.17 sec
/home/runner/work/dealii/dealii/tests/quick_tests/affinity.cc: In function ‘bool getaffinity(unsigned int&, unsigned int&)’:
/home/runner/work/dealii/dealii/tests/quick_tests/affinity.cc:53:42: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
   mask = *reinterpret_cast<int *>(&my_set);
                                          ^
cc1plus: all warnings being treated as errors

As mask is of type unsigned int, this should fix the problem.

Part of #13467 (comment).