ping example app throws lwip assert (#309)

Now LWIP_PLATFORM_ASSERT has been fixed we're seeing an assert for the
ping example app.
It sets the SO_RCVTIMEO socket option which is not enabled.

Fixes #308

pico_w/freertos/ping/lwipopts.h

@@ -16,6 +16,10 @@
 
 // not necessary, can be done either way
 #define LWIP_TCPIP_CORE_LOCKING_INPUT 1
+
+// ping_thread sets socket receive timeout, so enable this feature
+#define LWIP_SO_RCVTIMEO 1
 #endif
 
+
 #endif