Add cos_patch_trailer_check in PRESUBMIT.cfg.

cos_patch_trailer_check will check whether cos-patch
trailer is present or not. In case it is not present,
`repo upload` will fail.

BUG=b/173034990
TEST=manual test on changelist.
RELEASE_NOTE=None

Change-Id: I278bb937e4cfe61e941a703b7c394990ae187887
Reviewed-on: https://cos-review.googlesource.com/c/third_party/overlays/portage-stable/+/8663
Reviewed-by: Dexter Rivera <riverade@google.com>
Tested-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
diff --git a/PRESUBMIT.cfg b/PRESUBMIT.cfg
index 8bdbe9e..ba73fc2 100644
--- a/PRESUBMIT.cfg
+++ b/PRESUBMIT.cfg
@@ -8,6 +8,9 @@
 # Make sure RELEASE_NOTE field is present.
 release_note_field_check: true
 
+# Make sure cos_patch trailer is present.
+cos_patch_trailer_check: true
+
 stray_whitespace_check: false
 long_line_check: false
 cros_license_check: false