row_operations: suppress a gcc warning
row_operations: suppress a gcc warning

I couldn't get this one to budge with pragma suppressions, so I added a
default value instead.

Change-Id: I606958b25e8cd02bcf610a52e6c35b618f9f9788
Reviewed-on: http://gerrit.cloudera.org:8080/12943
Reviewed-by: Grant Henke <4cf7ebbe638391c4d27a10cf751b99bdbd1a1880@apache.org>
Tested-by: Kudu Jenkins

helifu/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu