Update code to be compatible with OIIO 2.0

There are some changes in API of OpenImageIO, but those are quite
simple to keep working with older and newer library versions.

Reviewers: brecht

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D4064

intern/cycles/blender/blender_python.cpp

@@ -500,7 +500,7 @@ static PyObject *osl_update_node_func(PyObject * /*self*/, PyObject *args)
 				socket_type = "NodeSocketString";
 				data_type = BL::NodeSocket::type_STRING;
 				if(param->validdefault)
-					default_string = param->sdefault[0];
+					default_string = param->sdefault[0].string();
 			}
 			else
 				continue;