smartattributes: Add missing default-disable file.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3182 44740490-163a-0410-bde0-09ae8108e29a

modules/smartattributes/default-disable

+This file indicates that the default state of this module
+is disabled. To enable, create a file named enable in the
+same directory as this file.