use struct Logger

Change-Id: I333425eb1acbad6b13a1b5926ef811f9ad3e5822
diff --git a/include/turn_cache.h b/include/turn_cache.h
index f3617c9..deeb919 100644
--- a/include/turn_cache.h
+++ b/include/turn_cache.h
@@ -31,7 +31,7 @@
 
 namespace dht {
 namespace log {
-class Logger;
+struct Logger;
 }
 }