[license] add SPDX headers to CMakeLists.txt files

CMakeLists.txt

bin/create_cmakelists.py

cmake/modules/CMakeLists.txt

doc/CMakeLists.txt

doc/doxygen/CMakeLists.txt

doc/handbook/CMakeLists.txt

dumux/CMakeLists.txt

dumux/adaptive/CMakeLists.txt

dumux/assembly/CMakeLists.txt

dumux/common/CMakeLists.txt

dumux/common/properties/CMakeLists.txt

dumux/common/typetraits/CMakeLists.txt

dumux/discretization/CMakeLists.txt

dumux/discretization/box/CMakeLists.txt

dumux/discretization/cellcentered/CMakeLists.txt

dumux/discretization/cellcentered/mpfa/CMakeLists.txt

...x/discretization/cellcentered/mpfa/omethod/CMakeLists.txt

dumux/discretization/cellcentered/tpfa/CMakeLists.txt

dumux/discretization/cvfe/CMakeLists.txt

dumux/discretization/facecentered/CMakeLists.txt