Fix a typo in ORT_API_CALL macro (_stdcall) (#25834)
### Description
Fixed the macro `ORT_API_CALL` by replacing `_stdcall` with `__stdcall`
### Motivation and Context
Recently, I found an issue that prevents ONNX Runtime from being built using the MinGW toolchain on Windows.
After investigating, I discovered that the ONNX Runtime C API header contains a typo in the `ORT_API_CALL` preprocessor macro.
It is incorrectly defined as `_stdcall` instead of the correct `__stdcall` (with two leading underscores).
This causes build failures on compilers like MinGW that are strict about this syntax.
