fix bug #2595: "loopif results

in NULL reference for incoming TCP packets". Loopif has to be configured
(using LWIP_LOOPIF_MULTITHREADING) to directly call netif->input()
(multithreading environments, e.g. netif->input() = tcpip_input()) or
putting packets on a list that is fed to the stack by calling loopif_poll()
(single-thread / NO_SYS / polling environment where e.g.
netif->input() = ip_input).

src/include/netif/loopif.h

@@ -34,6 +34,10 @@
 
 #include "lwip/netif.h"
 
+#if !LWIP_LOOPIF_MULTITHREADING
+void loopif_poll(struct netif *netif);
+#endif
+
 err_t loopif_init(struct netif *netif);
 
 #endif /* __NETIF_LOOPIF_H__ */