ping: move the echo responder into the ping plugin

Type: feature

Change-Id: I246346b82858e73b16d727e2106350bc0fc3c6f2
Signed-off-by: Neale Ranns <nranns@cisco.com>
diff --git a/src/vnet/ip/icmp4.h b/src/vnet/ip/icmp4.h
index 73bd1f4..e2a9567 100644
--- a/src/vnet/ip/icmp4.h
+++ b/src/vnet/ip/icmp4.h
@@ -55,6 +55,8 @@
   vnet_buffer (b)->ip.icmp.data = data;
 }
 
+extern vlib_node_registration_t ip4_icmp_input_node;
+
 #endif /* included_vnet_icmp4_h */
 
 /*