Fixed #10113 -- Ensured that joined fields mentioned in order_by() are included in the GROUP_BY clause on those backends that require it. Thanks to Koen Biermans <koen.biermans@werk.belgie.be> for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9788 bcc190cf-cafb-0310-a4f2-bffc1f526a37