fdo#70393: move redland to a subdir of external

Change-Id: Ia857f943b8f2915c857ec4c9fa363760699e5206
Reviewed-on: https://gerrit.libreoffice.org/6551
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>