Use [FLEXIBLE_ARRAY_MEMBER] not [1] in MultiSortSupportData.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: dde7fb7836c7428f79cb3cd88ca5febb802e767e
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2019-04-15T23:32:44Z
Releases: 12.0
Use [FLEXIBLE_ARRAY_MEMBER] not [1] in MultiSortSupportData.

This struct seems to have not gotten the word about preferred
coding style for variable-length arrays.

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