add a conditional branch for Windows only

Author: junkmd

stdlib/_ctypes.pyi

@@ -107,8 +107,10 @@ class CFuncPtr(_PointerLike, _CData):
     def __init__(self, callable: Callable[..., Any]) -> None: ...
     @overload
     def __init__(self, func_spec: tuple[str | int, CDLL], paramflags: tuple[_PF, ...] = ...) -> None: ...
-    @overload
-    def __init__(self, vtlb_index: int, name: str, paramflags: tuple[_PF, ...] = ..., iid: _Pointer[c_int] = ...) -> None: ...
+    if sys.platform == "win32":
+        @overload
+        def __init__(self, vtlb_index: int, name: str, paramflags: tuple[_PF, ...] = ..., iid: _Pointer[c_int] = ...) -> None: ...
+
     def __call__(self, *args: Any, **kwargs: Any) -> Any: ...
 
 class _CField: