netfilter: nft_tunnel: add the missing ERSPAN_VERSION nla_policy

ERSPAN_VERSION is an attribute parsed in kernel side, nla_policy
type should be added for it, like other attributes.

Fixes: af308b94 ("netfilter: nf_tables: add tunnel support")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>