fdo#70393: move postgresql to a subdir of external

Change-Id: Ibd7c9a4db0150bdb115b061e21587ca964dc2c84
Reviewed-on: https://gerrit.libreoffice.org/6549
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>