Enforcer rule to skip parent pom

[suztomo]

When applied to google-http-java-client's top level pom.xml (google-http-client-parent), the enforcer rule fails at the parent pom:

[INFO] --- maven-enforcer-plugin:3.0.0-M2:enforce (enforce-linkage-checker) @ google-http-client-parent ---
[WARNING] Rule 0: com.google.cloud.tools.dependencies.enforcer.LinkageCheckerRule failed with message:
Artifact com.google.http-client:google-http-client-parent:1.30.2-SNAPSHOT is not associated with a file. The linkage checker enforcer rule should be bound to the verify phase.

Because it is a pom artifact, there's no jar file associated with it.