Add *.swp to .gitignore

TEST=Check git status while another file is open for editing with vi
BUG=None

Change-Id: I1cf32da40c56580bcce5b6dccdfc2373127f431c
Signed-off-by: Greg Edelston <gredelston@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/1874231
Reviewed-by: Dana Goyette <dgoyette@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
diff --git a/.gitignore b/.gitignore
index 20665e7..478c2b4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 # Generated files
 *.pyc
 *~
+*.swp