Hot-fix ecpg regression test for missing ecpg_config.h inclusion.

I don't think this is really the best long-term answer, and in
particular it doesn't fix the pre-existing hazard in sqltypes.h.
But for the moment let's just try to make the buildfarm green again.

Discussion: https://postgr.es/m/151935568942.1461.14623890240535309745@wrigleys.postgresql.org