CRM-14680 - ensure custom tables included in sql statement

Fix for the Contribution Aggregate by Relationship report.

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* CRM-14680: Contribution Aggregate by Relationship report fails with custom contribution field
  https://issues.civicrm.org/jira/browse/CRM-14680
Merge pull request #3243 from jmcclelland/contribution-aggregate-report-custom-field

Contribution aggregate report custom field