HPCC-10454 Fix build break in windows - missing _API specifier

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5781 from ghalliday/issue10454

HPCC-10454 Fix build break in windows - missing _API specifier

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>