[gn build] Port 72402e8

Author: llvmgnsyncbot

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

@@ -1182,6 +1182,7 @@ if (current_toolchain == default_toolchain) {
       "__locale_dir/support/fuchsia.h",
       "__locale_dir/support/linux.h",
       "__locale_dir/support/netbsd.h",
+      "__locale_dir/support/newlib.h",
       "__locale_dir/support/no_locale/characters.h",
       "__locale_dir/support/no_locale/strtonum.h",
       "__locale_dir/support/windows.h",