OPENDJ-2941 Fix tools extension informations

The issue was a side effect of the configuration framework migration (see OPENDJ-1238).
With the legacy configuration framework, we ensured that the file admin/extension.manifest was present in the extension jar like snmp-mib2605.jar.
Now it seems that we check if the file META-INF/services/org.forgerock.opendj.config.AbstractManagedObjectDefinition exists in the extension jar.
In order to have the default snmp extension listed again in tools, add an empty file in snmp-mib2605.jar