php: add pdo_pgsql extension

Adds support for the PDO PostgreSQL extension to the PHP package.

[Thomas: slightly adjust Config.in comment to indicate that's it's the
"Postgresql driver" that needs (e)glibc and not just "Postgresql".]

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>