HPCC-10624 Fix incorrect warning when SKIP used in a ROLLUP

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5285 from ghalliday/issue10624

HPCC-10624 Fix incorrect warning when SKIP used in a ROLLUP

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>