fdo#70393: move jpeg to a subdir of external

Change-Id: I56be33efe0cc1deaf7a7e7ce6c6ea292dbe7786d
Reviewed-on: https://gerrit.libreoffice.org/6543
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>