netfilter: nft_meta_bridge: Remove the br_private.h header

nft_bridge_meta should not access the bridge internal API.

Signed-off-by: wenxu <wenxu@ucloud.cn>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>