package/postgresql: add configure and includedir-server output to pg_config

Some external packages call pg_config to determine the installed
PostgreSQL server includedir and configure options. Add this output to
Buildroots own pg_config, so these packages correctly compile.

Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>