.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>
diff --git a/.gitignore b/.gitignore
index 9edc243..bb49193 100644
--- a/.gitignore
+++ b/.gitignore
@@ -11,3 +11,4 @@
 scripts/newbitmaps/strings/localized_text/*/*.png
 target
 .idea
+*.swp