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

NuGet recently added functionality to support "build/" folders within packages which contain MSBuild .targets files that will be added to projects that install the package. (http://docs.nuget.org/docs/reference/support-for-native-projects)

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.