samples/net: Fix logging format error

When building the echo_client sample app, the following warning
is found:

echo-client.c:37:
echo_client/src/echo-client.c: In function 'send_ipv4_data':
include/logging/sys_log.h:108:20: warning: format '%d' expects argument
                                  of type 'int', but argument 7 has
                                  type 'size_t' ...

So, this patch changes %d by %zu to remove that warning.

Change-Id: I8a3822bca6d5988e531684d647de93bd77aabf9b
Signed-off-by: Flavio Santes <[email protected]>

Author: 1010101001010101

samples/net/echo_client/src/echo-client.c

@@ -414,7 +414,7 @@ static bool send_ipv4_data(struct net_context *udp)
 	len = net_buf_frags_len(send_buf);
 
 	NET_ASSERT_INFO(expecting_ipv4 == len,
-			"Data to send %d bytes, real len %d",
+			"Data to send %d bytes, real len %zu",
 			expecting_ipv4, len);
 
 	set_dst_addr(AF_INET, send_buf, &dst_addr);