Make SAML2\XML\md\RoleDescriptor abstract
Make SAML2\XML\md\RoleDescriptor abstract

The standard defines it as abstract, and we should reflect that in the code.

Signed-off-by: Jaime Pérez Crespo <97eacf607c7365bcf0d28d984fb632889cc59055@uninett.no>

simplesamlphp/saml2