qt5base: fix the PostgreSQL detection patch

The PostgreSQL detection patch added in commit
1b54fbc9 ("qt5base: fix postgresql
plugin compile") had a minor issue: it was still calling the host
pg_config to decided whether or not PostgreSQL was available.

[Thomas: improve commit log.]

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>