HPCC-10026 Support SIZEOF(ENUM)

One line fix with the addition of the runtime regression test, issue10026.ecl,
and it's associated key file.

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
Merge pull request #5033 from jamienoss/issue10026-sizeof-enum

HPCC-10026 Support SIZEOF(ENUM)

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>