Add missing fmgr.h include.

Andres Freund <andres@anarazel.de>

Commit: 00a5c4c17b76b2d68fab70ec0be0960590a3a7fe
Author: Andres Freund <andres@anarazel.de>
Date: 2019-08-16T22:19:50Z
Releases: 13.0
Add missing fmgr.h include.

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