fdo#70393: move openssl to a subdir of external

Change-Id: I10bf92b18ee5ad56f1b4fbee3e4008b35b822be4
Reviewed-on: https://gerrit.libreoffice.org/6547
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>