ecpg: Avoid C99-ism in newly-added test for Oracle compat

Michael Paquier <michael@paquier.xyz>

Commit: dbd25dd0b59c649981e0a26cc0f367b8c0368473
Author: Michael Paquier <michael@paquier.xyz>
Date: 2023-04-18T03:00:31Z
Releases: 11.20
ecpg: Avoid C99-ism in newly-added test for Oracle compat

Per buildfarm member mylodon.  As C99 declarations are allowed in v12~,
this is adjusted only on REL_11_STABLE.  Introduced by 9eb44bb.

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