Fix ignore_unspecified option for PropertySortOrder

This option was broken due to a typo in the `PropertySortOrder` linter.

Change-Id: Ie053012047ea9e023365d885a3d646e8ed3161af
Reviewed-on: http://gerrit.causes.com/41276
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>