Add 'struct ipxpcb' forward declaration to ipx_var.h. I had this in

the netperf branch but for some reason didn't trigger a build failure
locally when I merged to CVS and omitted it.  Presumably driver error.

Pointed out by:	cperciva, tinderbox

sys/netipx/ipx_var.h

@@ -78,6 +78,7 @@ extern const union ipx_host ipx_broadhost;
 
 struct ifnet;
 struct ipx_addr;
+struct ipxpcb;
 struct mbuf;
 struct thread;
 struct route;