[gn build] Port 854bbc5

Author: llvmgnsyncbot

llvm/utils/gn/secondary/llvm/lib/Target/X86/BUILD.gn

@@ -139,6 +139,7 @@ static_library("LLVMX86CodeGen") {
     "X86TileConfig.cpp",
     "X86VZeroUpper.cpp",
     "X86WinEHState.cpp",
+    "X86WinFixupBufferSecurityCheck.cpp",
   ]
 }