[llvm][llvm-exegesis] Fix unused private field warning on Windows

```
[5323/5730] Building CXX object tools\llvm-exegesis\lib\CMakeFiles\LLVMExegesis.dir\Error.cpp.obj
In file included from C:\Work\david.spickett\llvm-project\llvm\tools\llvm-exegesis\lib\Error.cpp:9:
C:\Work\david.spickett\llvm-project\llvm\tools\llvm-exegesis\lib/Error.h(76,7): warning: private field 'SignalNumber' is not used [-Wunused-private-field]
   76 |   int SignalNumber;
      |       ^
```

`SignalNumber` was only being used on Unixes.