[MINOR] indicate the proxy type in the logs after a loss of servers

When the last server goes down in a backend, indicate 'backend' or
'listener' in the log message depending on the type of the backend.
diff --git a/include/proto/proxy.h b/include/proto/proxy.h
index 968495f..3fa541a 100644
--- a/include/proto/proxy.h
+++ b/include/proto/proxy.h
@@ -31,6 +31,7 @@
 void pause_proxy(struct proxy *p);
 void pause_proxies(void);
 void listen_proxies(void);
+const char *proxy_type_str(struct proxy *proxy);
 
 
 #endif /* _PROTO_PROXY_H */