cmake: ignore files generated by CMake as run in Visual Studio

As of recent Visual Studio versions, CMake support is built-in: https://docs.microsoft.com/en-us/cpp/build/cmake-projects-in-visual-studio?view=vs-2019 All that needs to be done is to open the worktree as a folder, and Visual Studio will find the `CMakeLists.txt` file and automatically generate the project files. Let's ignore the entirety of those generated files. Helped-by: Đoàn Trần Công Danh <congdanhqx@gmail.com> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>