Add *.VC.opendb for UnrealEngine

Add *.VC.opendb to the .gitignore for the UnrealEngine for Visual Studio 2015 Update 1 users. This reflects the changes in the VisualStudio.gitignore made in https://github.com/github/gitignore/pull/1778 and https://github.com/github/gitignore/pull/1783 but is more specifically targeted at solving the issue http://stackoverflow.com/questions/34319008/git-issue-with-visual-studio-2015-update-1 .