fdo#70393: move unixODBC to a subdir of external

Change-Id: I191b75802eba781fc62ab21cd41918983a6efdb0
Reviewed-on: https://gerrit.libreoffice.org/6569
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>