[gn build] Port 76985fd

Author: llvmgnsyncbot

llvm/utils/gn/secondary/llvm/lib/MC/BUILD.gn

@@ -14,6 +14,7 @@ static_library("MC") {
   sources = [
     "ConstantPools.cpp",
     "DXContainerPSVInfo.cpp",
+    "DXContainerRootSignature.cpp",
     "ELFObjectWriter.cpp",
     "GOFFObjectWriter.cpp",
     "MCAsmBackend.cpp",