move PartitionDispatchData definition to execPartition.c

Hi.

I think we may have simply forgotten to do $subject in the following commit.

commit da6f3e45ddb68ab3161076e120e7c32cfd46d1db
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date:   Sat Apr 14 21:12:14 2018 -0300

    Reorganize partitioning code

Attached a patch.

Thanks,
Amit