.gitignore add file from frameworks/kconfig

Ignore CMakeLists.txt.user and compile_commands.json created by kdesrc-build/ecm.
https://community.kde.org/Get_Involved/development/Easy#Just_running_kdesrc-build_should_not_create_%22Unstaged_Changes%22_in_the_local_clone_of_a_KDE_git_repo