Add 'OwlBot Post Processor' as a required check

.github/sync-repo-settings.yaml

@@ -49,6 +49,7 @@ branchProtectionRules:
     - 'Kokoro system-3.8'
     - 'cla/google'
     - 'Conformance / Async v3 Client / Python 3.8'
+    - 'OwlBot Post Processor'
 # List of explicit permissions to add (additive only)
 permissionRules:
     # Team slug to add to repository permissions