Unignore "packages/*/build" in VisualStudio.gitignore.

NuGet's support added functionality to support "build/" folders within packages which contain MSBuild .targets files that will be added to projects that install the package.

While most people hopefully use package restore, the default VisualStudio.gitignore assumes you are going to check in the packages folder. However, it has a line that ignores any "build/" folder, meaning some of the package content will not be checked in.

The easiest solution here seems to be to "unignore" the "build/" folder when encountered inside the NuGet "packages/" folder. Which is what this PR does.