Move PartitionDispatchData struct definition to execPartition.c
There's no reason to expose the struct definition, so don't.
Author: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>
Discussion: https://postgr.es/m/
d3fa24c1-bc65-7133-81df-
6474387ccc4f@lab.ntt.co.jp