HPCC-11444 Fix signature of FlushingJsonBuffer::startDataset

Base rather than derived version was being called because the
signatures no longer matched.

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Merge pull request #5895 from afishbeck/jsonDatasetObjectRegression

HPCC-11444 Fix signature of FlushingJsonBuffer::startDataset

Reviewed-By: Gordon Smith <gordon.smith@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>