workflows/dco-report: drop permissions

Ensure the workflow doesn't get write access in forks configured to give
it by default.

Update for openslide/.github#4.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>

Author: bgilbert

.github/workflows/dco-report.yml

@@ -6,6 +6,9 @@ on:
     types:
       - completed
 
+permissions:
+  contents: none
+
 jobs:
   comment:
     name: Organization