PPP, sifnpmode() is only useful if on demand is supported, build out if not

We don't support PPP on demand, don't build sifnpmode() which is only
useful for on demand if on demand is not supported.

src/include/netif/ppp/ppp_impl.h

@@ -462,7 +462,9 @@ int sif6up(ppp_pcb *pcb);
 int sif6down (ppp_pcb *pcb);
 #endif /* PPP_IPV6_SUPPORT */
 
+#if DEMAND_SUPPORT
 int sifnpmode(ppp_pcb *pcb, int proto, enum NPmode mode);
+#endif /* DEMAND_SUPPORt */
 
 void netif_set_mtu(ppp_pcb *pcb, int mtu);
 int netif_get_mtu(ppp_pcb *pcb);