workflow: sign commits (#4526)

This doesn't do much, but at least it doesn't litter PRs with non-signed
commits. This should keep the DCO clean(er)

Signed-off-by: Miek Gieben <miek@miek.nl>

.github/workflows/whitespace.yml

@@ -31,6 +31,6 @@ jobs:
         run: |
           git add .
           if output=$(git status --porcelain) && [ ! -z "$output" ]; then
-            git commit -m 'auto remove trailing whitespaces'
+            git commit -s -m 'auto remove trailing whitespaces'
             git push
           fi