[gn build] port b973e2e

Author: nico

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

@@ -69,6 +69,7 @@ copy("include") {
   sources = [
     "__availability",
     "__bit_reference",
+    "__bits",
     "__bsd_locale_defaults.h",
     "__bsd_locale_fallbacks.h",
     "__debug",