[gn] port 3694697

Author: nico

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

@@ -264,6 +264,7 @@ copy("Headers") {
     "stdarg.h",
     "stdatomic.h",
     "stdbool.h",
+    "stdckdint.h",
     "stddef.h",
     "stdint.h",
     "stdnoreturn.h",