API: Add service definitions for events and singleton messages (second attempt)

Based on https://gerrit.fd.io/r/#/c/10920/

Updates service definition in stats.api with correct reply message names.

Change-Id: I3282bee5304e667e23bc1fab3f43d967a50d880d
Signed-off-by: Marek Gradzki <mgradzki@cisco.com>
diff --git a/src/vnet/dhcp/dhcp.api b/src/vnet/dhcp/dhcp.api
index 721a1be..975aa6e 100644
--- a/src/vnet/dhcp/dhcp.api
+++ b/src/vnet/dhcp/dhcp.api
@@ -109,6 +109,10 @@
   u8 host_mac[6];
 };
 
+service {
+  rpc dhcp_client_config returns dhcp_client_config_reply events dhcp_compl_event;
+};
+
 /** \brief Dump DHCP proxy table
     @param client_index - opaque cookie to identify the sender
     @param True for IPv6 proxy table