netfilter: nft_tproxy: Move nf_tproxy_assign_sock() to nf_tproxy.h

This function is also necessary to implement nft tproxy support

Fixes: 45ca4e0c ("netfilter: Libify xt_TPROXY")
Signed-off-by: Máté Eckl <ecklm94@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>