[gn build] Port 56e3fc4

Author: llvmgnsyncbot

llvm/utils/gn/secondary/llvm/lib/Frontend/HLSL/BUILD.gn

@@ -7,6 +7,8 @@ static_library("HLSL") {
   sources = [
     "CBuffer.cpp",
     "HLSLResource.cpp",
-    "HLSLRootSignatureUtils.cpp",
+    "HLSLRootSignature.cpp",
+    "RootSignatureMetadata.cpp",
+    "RootSignatureValidations.cpp",
   ]
 }