Make sure we prefer the bundled flatbuffers copy.

Index: modules/dnn/CMakeLists.txt
--- modules/dnn/CMakeLists.txt.orig
+++ modules/dnn/CMakeLists.txt
@@ -146,7 +146,9 @@ if(TARGET ocv.3rdparty.flatbuffers AND OPENCV_DNN_TFLI
   list(APPEND libs ocv.3rdparty.flatbuffers)
   list(APPEND fw_hdrs "${CMAKE_CURRENT_LIST_DIR}/misc/tflite/schema_generated.h")
   list(APPEND fw_inc "${CMAKE_CURRENT_LIST_DIR}/misc/tflite")
-
+  if(WITH_FLATBUFFERS)
+    list(APPEND fw_inc "${OpenCV_SOURCE_DIR}/3rdparty/flatbuffers/include")
+  endif()
   # Schema is generated by this command:
   #add_custom_command(
   #      OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/schema_generated.h"
