.gitignore: ignore swap files of vim BUG=none BRANCH=none TEST=none Signed-off-by: Hsin-Te Yuan <yuanhsinte@google.com> Change-Id: Ia29490c6fb8bf02a98ef09b1d284369802a2b750 Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3719468 Reviewed-by: Yu-Ping Wu <yupingso@chromium.org> Commit-Queue: Hsuan Ting Chen <roccochen@chromium.org> Reviewed-by: Hsuan Ting Chen <roccochen@chromium.org>