fix(ml): make MNE_USE_ONNXRUNTIME PUBLIC to fix ABI mismatch

The compile definition was PRIVATE, so only libmne_ml saw it.
Consumers (e.g. test_ml_models) compiled ml_onnx_model.h without
the define and got a smaller class layout (missing ORT member
variables), causing stack corruption ('stack smashing detected').

Author: chdinh

src/libraries/ml/CMakeLists.txt

@@ -53,7 +53,7 @@ target_link_libraries(${PROJECT_NAME} PRIVATE
 # ONNX Runtime (optional — enabled when USE_ONNXRUNTIME=ON and the package was found)
 if(TARGET onnxruntime::onnxruntime)
     target_link_libraries(${PROJECT_NAME} PRIVATE onnxruntime::onnxruntime)
-    target_compile_definitions(${PROJECT_NAME} PRIVATE MNE_USE_ONNXRUNTIME)
+    target_compile_definitions(${PROJECT_NAME} PUBLIC MNE_USE_ONNXRUNTIME)
     message(STATUS "[mne_ml] ONNX Runtime enabled")
 
     # Copy the ONNX Runtime shared library next to mne_ml so the runtime linker