gitignore: Add kbfs_sync_cache folder to gitignore.
gitignore: Add kbfs_sync_cache folder to gitignore.

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs