Don't include rel.h when relcache.h is sufficient

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: 4901b2f49536f439f79d067a9b30a7a837d6c72b
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2015-08-11T16:03:14Z
Releases: 9.6.0
Don't include rel.h when relcache.h is sufficient

Trivial change to reduce exposure of rel.h.

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