[gn build] Port cab1ae9

Author: llvmgnsyncbot

llvm/utils/gn/secondary/clang/lib/Headers/BUILD.gn

@@ -209,6 +209,7 @@ copy("Headers") {
     "hexagon_types.h",
     "hlsl.h",
     "hlsl/hlsl_basic_types.h",
+    "hlsl/hlsl_detail.h",
     "hlsl/hlsl_intrinsics.h",
     "hresetintrin.h",
     "htmintrin.h",