PRESUMBIT: enable signoff_check

The default in pre-upload.py was changed to warn about Signed-Off-By
lines: https://crrev.com/c/1917972.
BRANCH=none
BUG=none
TEST=repo upload --cbr .

Change-Id: Ie54b95251a751134c8824d3720e2fe73ef79fbc9
Signed-off-by: Jett Rink <jettrink@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1926520
Reviewed-by: Joel Kitching <kitching@chromium.org>
diff --git a/PRESUBMIT.cfg b/PRESUBMIT.cfg
index 55b4b57..8770083 100644
--- a/PRESUBMIT.cfg
+++ b/PRESUBMIT.cfg
@@ -1,5 +1,6 @@
 [Hook Overrides]
 branch_check: true
+signoff_check: true
 
 # We are using Linux style indentation with tabs
 # The indentation is checked by checkpatch not the python script