[cmake] Add interface keyword PUBLIC where missing.

cmake/modules/AddGMPFlags.cmake

cmake/modules/AddMETISFlags.cmake

cmake/modules/AddPTScotchFlags.cmake

cmake/modules/AddParMETISFlags.cmake

cmake/modules/AddQuadMathFlags.cmake

cmake/modules/AddSuiteSparseFlags.cmake

cmake/modules/AddVcFlags.cmake

cmake/modules/DuneMPI.cmake

cmake/modules/DuneMacros.cmake

cmake/modules/DuneTestMacros.cmake

cmake/modules/FindTBB.cmake

doc/comm/CMakeLists.txt

dune/common/parallel/benchmark/CMakeLists.txt

dune/common/test/CMakeLists.txt

dune/python/test/CMakeLists.txt

python/dune/common/module.py