netfilter: fix warning in net/netfilter/nf_conntrack_proto_tcp.c

fix this warning:

  net/netfilter/nf_conntrack_proto_tcp.c: In function \u2018tcp_in_window\u2019:
  net/netfilter/nf_conntrack_proto_tcp.c:491: warning: unused variable \u2018net\u2019
  net/netfilter/nf_conntrack_proto_tcp.c: In function \u2018tcp_packet\u2019:
  net/netfilter/nf_conntrack_proto_tcp.c:812: warning: unused variable \u2018net\u2019

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>