[license] add SPDX headers to CMake modules

cmake/modules/AddGstatFileLinks.cmake

cmake/modules/AddInputFileLinks.cmake

cmake/modules/AddKokkosFlags.cmake

cmake/modules/AddOpenMPFlags.cmake

cmake/modules/DumuxDoxygen.cmake

cmake/modules/DumuxMacros.cmake

cmake/modules/DumuxTestMacros.cmake

cmake/modules/FindGLPK.cmake

cmake/modules/FindGmsh.cmake

cmake/modules/FindGstat.cmake

cmake/modules/FindNLOPT.cmake

cmake/modules/FindPVPython.cmake