Add *build* to the auto-generated .gitignore

<Issue>
I'm not sure since which version Qt Creator started to create a build directory inside the source directory, but in 14.0.0, it generates a build directory under the source directory by default.

If a user selects "Git" when creating a project, Qt Creator automatically creates .git and .gitignore, under the source directory.

Hence, when a user uses, for example, git add . after he/she has configured the project with cmake or built the app, all the build artifacts inside the build directory are staged.

I don't think this is what most of the users want, therefore Qt Creator should automatically add an entry to let git ignore the contents in the build directory to the generated .gitignore.