Revert "Add 3rdparty_upstream and 3rdparty to .gitignore"

This reverts commit e13b70626cdc593d9cfad48b9ffff6d4c4993585
since it seems to cause problems with the init-repository script.

Change-Id: Ie8a038041b3a17dcb0bb1f629285205a328a6819
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>