use struct Logger

Change-Id: I333425eb1acbad6b13a1b5926ef811f9ad3e5822
diff --git a/include/ice_transport.h b/include/ice_transport.h
index d1bba38..ca4b086 100644
--- a/include/ice_transport.h
+++ b/include/ice_transport.h
@@ -32,7 +32,7 @@
 
 namespace dht {
 namespace log {
-class Logger;
+struct Logger;
 }
 }