[Developer] Update to '.gitignore' to not track user specific 'cpp/Brewfile.lock.json' file

[asfimport]

In the developer guides for Python, there is a suggestion for users on macOS to use Homebrew to install all dependencies required for building Arrow C++. This creates a 'cpp/Brewfile.lock.json' file is specific to the system it sits on.

It would be desirable for this not to be tracked by version control. To prevent this accidental addition, perhaps it should be ignored in the gitignore file for the respository

Environment: macOS-10.15.3
Reporter: Tarek Allam / @tallamjr
Assignee: Tarek Allam / @tallamjr

PRs and other links:

• GitHub Pull Request #6494

_{Note: This issue was originally created as ARROW-7949. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 6494
#6494