collectd: add configure hint for pg_config

Fixes [1] in case postgresql-devel package is installed on the host:

    libpq . . . . . . . . no (libpq-fe.h not found)
    postgresql  . . . . . no (dependency error)
    configure: error: "Some plugins are missing dependencies - see the summary above for details"

Otherwise fixes the following configure warning:

	configure: WARNING: pg_config returned with status 127

[1] http://autobuild.buildroot.net/results/336/336b3e932be245faa04969af960702af672916dc



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