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>

.gitignore

@@ -241,3 +241,4 @@ Release/
 /git.VC.VC.opendb
 /git.VC.db
 *.dSYM
+/contrib/buildsystems/out