[gn build] Port 5ab944a

Author: llvmgnsyncbot

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

@@ -320,6 +320,7 @@ copy("Headers") {
     "stdatomic.h",
     "stdbool.h",
     "stdckdint.h",
+    "stdcountof.h",
     "stddef.h",
     "stdint.h",
     "stdnoreturn.h",