fdo#70393: move np_sdk to a subdir of external

Change-Id: Iafba7ff0d0a4b163f9d99195cab36c9968e0bd73
Reviewed-on: https://gerrit.libreoffice.org/6558
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>