Fix compilation error found in tflite test (#3820)

ps. https://github.com/bytecodealliance/wasm-micro-runtime/pull/3817

core/iwasm/libraries/wasi-nn/cmake/Findcjson.cmake

@@ -4,13 +4,21 @@
 include(FetchContent)
 
 set(CJSON_SOURCE_DIR "${WAMR_ROOT_DIR}/core/deps/cjson")
-
-FetchContent_Declare(
-  cjson
-  GIT_REPOSITORY https://github.com/DaveGamble/cJSON.git
-  GIT_TAG        v1.7.18
-  SOURCE_DIR     ${CJSON_SOURCE_DIR}
-)
+if(EXISTS ${CJSON_SOURCE_DIR})
+  message("Use existed source code under ${CJSON_SOURCE_DIR}")
+  FetchContent_Declare(
+    cjson
+    SOURCE_DIR     ${CJSON_SOURCE_DIR}
+  )
+else()
+  message("download source code and store it at ${CJSON_SOURCE_DIR}")
+  FetchContent_Declare(
+    cjson
+    GIT_REPOSITORY https://github.com/DaveGamble/cJSON.git
+    GIT_TAG        v1.7.18
+    SOURCE_DIR     ${CJSON_SOURCE_DIR}
+  )
+endif()
 
 set(ENABLE_CJSON_TEST OFF CACHE INTERNAL "Turn off tests")
 set(ENABLE_CJSON_UNINSTALL OFF CACHE INTERNAL "Turn off uninstall to avoid targets conflict")