gitignore: ignore VS 2015 *.VC.opendb files

These files are created by VS 2015 and should be ignored by git.

PR-URL: https://github.com/nodejs/node/pull/6070


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>