chore: make owlbot check required (#186)

Author: Neenu1995

.github/sync-repo-settings.yaml

@@ -39,6 +39,7 @@ branchProtectionRules:
     - "units (11)"
     - "Kokoro - Test: Integration"
     - "cla/google"
+    - "OwlBot Post Processor"
 # List of explicit permissions to add (additive only)
 permissionRules:
 - team: yoshi-admins