[tcp] Ensure FIN is actually sent if connection is closed while idle

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp.c

 
 		tcp->tcp_state |= TCP_STATE_SENT ( TCP_FIN );
 		tcp_dump_state ( tcp );
+		process_add ( &tcp->process );
 
 		/* Add a pending operation for the FIN */
 		pending_get ( &tcp->pending_flags );