Fix sys_tree macro stubs with no arguments.

This fixes Visual Studio compiler warnings like this one:
"warning C4003: not enough arguments for function-like
macro invocation 'G_MSGS_DROPPED_INC'"

Signed-off-by: Sigmund Vik <sigmund_vik@yahoo.com>

src/sys_tree.h

@@ -54,10 +54,10 @@ extern unsigned int g_connection_count;
 #define G_MSGS_SENT_INC(A)
 #define G_PUB_MSGS_RECEIVED_INC(A)
 #define G_PUB_MSGS_SENT_INC(A)
-#define G_MSGS_DROPPED_INC(A)
-#define G_CLIENTS_EXPIRED_INC(A)
-#define G_SOCKET_CONNECTIONS_INC(A)
-#define G_CONNECTION_COUNT_INC(A)
+#define G_MSGS_DROPPED_INC()
+#define G_CLIENTS_EXPIRED_INC()
+#define G_SOCKET_CONNECTIONS_INC()
+#define G_CONNECTION_COUNT_INC()
 
 #endif