package/apr-util: add optional support for postgresql

apr-util contains optional support for postgresql:

$ ls -1 output/target/usr/lib/apr-util-1/apr_dbd_pg*
output/target/usr/lib/apr-util-1/apr_dbd_pgsql-1.so
output/target/usr/lib/apr-util-1/apr_dbd_pgsql.la
output/target/usr/lib/apr-util-1/apr_dbd_pgsql.so

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>