[fix](fe) Fix enable_nereids_planner forward not take effect (apach…

…e#26782)

* The java reflection method `getFields()` only return public fields,
  but enable_nereids_planner is private

fe/fe-core/src/main/java/org/apache/doris/qe/SessionVariable.java

@@ -2502,8 +2502,9 @@ public Map<String, String> getForwardVariables() {
      **/
     public void setForwardedSessionVariables(Map<String, String> variables) {
         try {
-            Field[] fields = SessionVariable.class.getFields();
+            Field[] fields = SessionVariable.class.getDeclaredFields();
             for (Field f : fields) {
+                f.setAccessible(true);
                 VarAttr varAttr = f.getAnnotation(VarAttr.class);
                 if (varAttr == null || !varAttr.needForward()) {
                     continue;