use struct Logger

Change-Id: I333425eb1acbad6b13a1b5926ef811f9ad3e5822
diff --git a/include/certstore.h b/include/certstore.h
index 1f85000..097a543 100644
--- a/include/certstore.h
+++ b/include/certstore.h
@@ -29,7 +29,7 @@
 
 namespace dht {
 namespace log {
-class Logger;
+struct Logger;
 }
 }
 
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;
 }
 }
 
diff --git a/include/multiplexed_socket.h b/include/multiplexed_socket.h
index e243946..b440891 100644
--- a/include/multiplexed_socket.h
+++ b/include/multiplexed_socket.h
@@ -30,7 +30,7 @@
 
 namespace dht {
 namespace log {
-class Logger;
+struct Logger;
 }
 }
 
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;
 }
 }
 
diff --git a/src/turn/turn_transport.h b/src/turn/turn_transport.h
index 1802073..b872454 100644
--- a/src/turn/turn_transport.h
+++ b/src/turn/turn_transport.h
@@ -27,7 +27,7 @@
 
 namespace dht {
 namespace log {
-class Logger;
+struct Logger;
 }
 }