Add signoff_check in PRESUBMIT.cfg for kernel repository.

This CL enables the check to have `Signed-off` field present
in Kernel commit.

BUG=b/173034990
TEST=Tested on a changelist
RELEASE_NOTE=None

Change-Id: I16dc50152705c33b86468eb245c7c9af647ca92f
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/8940
Reviewed-by: Roy Yang <royyang@google.com>
Tested-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
(cherry picked from commit 1457afba44c9598049ddef1aa99aef023d6be7cc)
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/8981
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-by: Dexter Rivera <riverade@google.com>