[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.

Author: DavidSpickett

llvm/tools/llvm-exegesis/lib/Error.cpp

@@ -43,6 +43,8 @@ void SnippetSignal::log(raw_ostream &OS) const {
   OS << "snippet crashed while running";
 #ifdef LLVM_ON_UNIX
   OS << ": " << strsignal(SignalNumber);
+#else
+  (void)SignalNumber;
 #endif // LLVM_ON_UNIX
 }