fix build break if enable CONFIG_NET_IPv6 only
In file included from tcp/tcp_sendfile.c:38:
tcp/tcp_sendfile.c: In function ‘sendfile_eventhandler’:
tcp/tcp_sendfile.c:173:27: error: ‘struct tcp_conn_s’ has no member named ‘domain’
173 | DEBUGASSERT(conn->domain == PF_INET6);
| ^~
Signed-off-by: chao an <anchao@xiaomi.com>