fix validation error for OOXML chart export, related fdo#67300

Change-Id: I7d74439bba3b912fe9b81ebeca865298bc32d71e
diff --git a/oox/source/export/chartexport.cxx b/oox/source/export/chartexport.cxx
index 9c004e0..c372151 100644
--- a/oox/source/export/chartexport.cxx
+++ b/oox/source/export/chartexport.cxx
@@ -1278,8 +1278,11 @@ void ChartExport::exportPieChart( Reference< chart2::XChartType > xChartType )
    sal_Int32 nAttachedAxis = AXIS_PRIMARY_Y;
    exportSeries( xChartType, nAttachedAxis );

    // firstSliceAng
    exportFirstSliceAng( );
    if( !mbIs3DChart )
    {
        // firstSliceAng
        exportFirstSliceAng( );
    }

    pFS->endElement( FSNS( XML_c, nTypeId ) );
}