HPCC-9105 Add missing const which was causing a compile error.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4207 from ghalliday/issue9105

HPCC-9105 Add missing const which was causing a compile error.

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>