platform: log when stats are caught

Write a log message when statcatcher receives an ipv6 multicast packet
from the ONU.

Change-Id: I96d2d0f30ab13c19916b166b49d6fa61bd55cc88
diff --git a/cmds/statcatcher.cc b/cmds/statcatcher.cc
index 184c725..64db2b6 100644
--- a/cmds/statcatcher.cc
+++ b/cmds/statcatcher.cc
@@ -119,6 +119,8 @@
     if (recvsize < 0) {
       perror("Failed to receive data on socket.\n");
       exit(1);
+    } else {
+      fprintf(stderr, "received %d bytes\n", recvsize);
     }
     pkt.resize(recvsize);