message_ in TException can't be const or subclasses break

Reviewed By: yishan


git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@664898 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/lib/cpp/src/Thrift.h b/lib/cpp/src/Thrift.h
index 89b37d0..6174d46 100644
--- a/lib/cpp/src/Thrift.h
+++ b/lib/cpp/src/Thrift.h
@@ -32,7 +32,7 @@
   }
 
 private:
-  const std::string message_;
+  std::string message_;
 
 };