HPCC-9173 Ubuntu 13.04 Build Fails

MIssing deffintion for PRIO_PROCESS

Signed-off-by: Gordon Smith <gordon.smith@lexisnexis.com>
Merge pull request #4260 from GordonSmith/HPCC-9173

HPCC-9173 Ubuntu 13.04 Build Fails

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>