net:lwip: Add debug line to net-lwip

When debugging the LWIP NFS implementation this debug line helped to
show the cause of an error. This could be useful to someone in the
future.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>

net/lwip/net-lwip.c

@@ -297,6 +297,7 @@ static struct pbuf *alloc_pbuf_and_copy(uchar *data, int len)
 	/* We allocate a pbuf chain of pbufs from the pool. */
 	p = pbuf_alloc(PBUF_RAW, len, PBUF_POOL);
 	if (!p) {
+		debug("Failed to allocate pbuf !!!!!\n");
 		LINK_STATS_INC(link.memerr);
 		LINK_STATS_INC(link.drop);
 		return NULL;