fdo#70393: move nss to a subdir of external

Change-Id: Ibbeb6b284c6c5fb9bba4ad52abed69321bba90d8
Reviewed-on: https://gerrit.libreoffice.org/6545
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>