Add ignore-windows to a test.

This test has an expected stderr containing text like this:

```
help: the following other types implement trait `std::error::Error`:
             ...
           and 43 others
```

However, the number 43 is platform-specific; on Windows, there are two
additional types, `InvalidHandleError` and `NullHandleError`, and the
number of 45. So for now, disable this test on Windows.

Author: sunfishcode

src/test/ui/coercion/coerce-issue-49593-box-never.rs

@@ -1,4 +1,5 @@
 // revisions: nofallback fallback
+// ignore-windows - the number of `Error` impls is platform-dependent
 //[fallback] check-pass
 //[nofallback] check-fail