[cmake] allow user/module specific ignore files for cpack
the CPACK_SOURCE_IGNORE_FILES is hard wired in DuneMacros.cmake. While this is a good idea to ensure a working setup in most cases, it blocks module specific ignores.
We add an additional list CPACK_SOURCE_IGNORE_FILES_EXTRA that can be specified by the user and will by appended to the CPACK_SOURCE_IGNORE_FILES list.