fdo#70393: move ucpp to a subdir of external

Change-Id: I05419939ca0d6c55d24088d4b4adf3e262e50083
Reviewed-on: https://gerrit.libreoffice.org/6553
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>