netfilter: Fix SIP conntrack build with NAT disabled.

Reported by Ingo Molnar.

The SIP helper is also useful without NAT. This patch adds an ifdef
around the RTP call optimization for NATed clients.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>