[llvm] [DirectX][NFC] Refactor `DXILRootSignature` to follow the same pattern as other analysis (PR #146783)

[Finn Plummer via llvm-commits]

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@@ -83,3 +106,4 @@ class RootSignatureAnalysisPrinter
 
 } // namespace dxil
 } // namespace llvm
+#endif
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inbelic wrote:

```suggestion
#endif // LLVM_LIB_TARGET_DIRECTX_DXILROOTSIGNATURE_H
```

https://github.com/llvm/llvm-project/pull/146783