build: Ensure all required checks run on merge_group

And clean up some of the whitespace formatting so that the "name" and
"on" are grouped away from the "jobs".

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

.github/workflows/dco.yaml

@@ -1,6 +1,6 @@
 name: DCO
-on:
-  pull_request:
+on: [pull_request, merge_group]
+
 jobs:
   check:
     runs-on: ubuntu-latest