Added check to the staticfiles app to make sure the STATIC_ROOT setting isn't...

Added check to the staticfiles app to make sure the STATIC_ROOT setting isn't accidentally added to the STATICFILES_DIRS setting. Thanks for the suggestion, SmileyChris and harijay.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15376 bcc190cf-cafb-0310-a4f2-bffc1f526a37