Fix ImageProcessor OSS build (pytorch#20010)

Fix linker error in image processor in OSS build. Internal buck has this
flag defined.

Author: metascroy

extension/image/image_processor.cpp

@@ -12,6 +12,7 @@
 #include <cstring>
 #include <memory>
 
+#define STB_IMAGE_RESIZE_IMPLEMENTATION
 #include <stb_image_resize.h>
 
 #include <executorch/runtime/core/error.h>

test/run_oss_cpp_tests.sh

@@ -47,7 +47,7 @@ build_executorch() {
     -DEXECUTORCH_BUILD_KERNELS_QUANTIZED=ON \
     -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON \
     -DEXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR=ON \
-    -DEXECUTORCH_BUILD_EXTENSION_IMAGE=OFF \
+    -DEXECUTORCH_BUILD_EXTENSION_IMAGE=ON \
     -DEXECUTORCH_BUILD_EXTENSION_MODULE=ON \
     -DEXECUTORCH_BUILD_EXTENSION_NAMED_DATA_MAP=ON \
     -DEXECUTORCH_BUILD_EXTENSION_LLM=ON \