chore: make owlbot check required (#116)

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Fixes #<issue_number_goes_here> ☕️

.github/sync-repo-settings.yaml

@@ -16,6 +16,7 @@ branchProtectionRules:
       - units (11)
       - 'Kokoro - Test: Integration'
       - cla/google
+      - OwlBot Post Processor
   - pattern: java7
     isAdminEnforced: true
     requiredApprovingReviewCount: 1
@@ -45,6 +46,7 @@ branchProtectionRules:
       - units (11)
       - 'Kokoro - Test: Integration'
       - cla/google
+      - OwlBot Post Processor
 permissionRules:
   - team: yoshi-admins
     permission: admin