[refactor] add reference path for source file from impala (#9115)

According to the requirements of the APLv2, the referenced code needs to be marked with the path of the source code.

be/src/exec/exec_node.cpp

@@ -14,6 +14,9 @@
 // KIND, either express or implied.  See the License for the
 // specific language governing permissions and limitations
 // under the License.
+// This file is copied from
+// https://github.com/apache/impala/blob/branch-2.9.0/be/src/exec/exec-node.cpp
+// and modified by Doris
 
 #include "exec/exec_node.h"