build: make OwlBot check required (#1354)

Author: chingor13

synthtool/gcp/templates/java_library/.github/sync-repo-settings.yaml

@@ -37,6 +37,7 @@ branchProtectionRules:
   - "units (11)"
   - "Kokoro - Test: Integration"
   - "cla/google"
+  - "OwlBot Post Processor"
 # List of explicit permissions to add (additive only)
 permissionRules:
 - team: yoshi-admins