HPCC-8566 Fix XML Schema generation skips top level attributes

Attributes defined in the top level record structure were being
ignored by the schema generator.

Signed-off-by: Anthony Fishbeck <Anthony.Fishbeck@lexisnexis.com>
Merge pull request #3834 from afishbeck/toplevel_record_attributes8566

HPCC-8566 Fix XML Schema generation skips top level attributes

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>