Fix compilation error found in tflite test (#3820)

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

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

@@ -4,13 +4,21 @@
 include(FetchContent)
 
 set(LLAMA_SOURCE_DIR "${WAMR_ROOT_DIR}/core/deps/llama.cpp")
-
-FetchContent_Declare(
-  llamacpp
-  GIT_REPOSITORY https://github.com/ggerganov/llama.cpp.git
-  GIT_TAG        b3573
-  SOURCE_DIR     ${LLAMA_SOURCE_DIR}
-)
+if(EXISTS ${LLAMA_SOURCE_DIR})
+  message("Use existed source code under ${LLAMA_SOURCE_DIR}")
+  FetchContent_Declare(
+    llamacpp
+    SOURCE_DIR     ${LLAMA_SOURCE_DIR}
+  )
+else()
+  message("download source code and store it at ${LLAMA_SOURCE_DIR}")
+  FetchContent_Declare(
+    llamacpp
+    GIT_REPOSITORY https://github.com/ggerganov/llama.cpp.git
+    GIT_TAG        b3573
+    SOURCE_DIR     ${LLAMA_SOURCE_DIR}
+  )
+endif()
 
 set(LLAMA_BUILD_TESTS OFF)
 set(LLAMA_BUILD_EXAMPLES OFF)