fix(测试跟踪): 用例导出脑图格式缺少前提和操作步骤的问题

--bug=1015760 --user=宋昌昌 【测试跟踪】github#16800，功能用例导出xmind 缺少前提和操作步骤 https://www.tapd.cn/55049933/s/1222394

backend/src/main/java/io/metersphere/base/mapper/ext/ExtTestCaseMapper.xml

@@ -252,6 +252,7 @@
         project_version.name as versionName,
         project_version.id as versionId,
         test_case.last_execute_result,
+        test_case.prerequisite, test_case.steps,
         <if test="request.selectFields != null and request.selectFields.size() > 0">
             <foreach collection="request.selectFields" item="field" separator=",">
                 ${field}