[gn build] Port e983a65

Author: llvmgnsyncbot

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

@@ -155,6 +155,9 @@ if (current_toolchain == default_toolchain) {
       "__bits",
       "__bsd_locale_defaults.h",
       "__bsd_locale_fallbacks.h",
+      "__charconv/chars_format.h",
+      "__charconv/from_chars_result.h",
+      "__charconv/to_chars_result.h",
       "__compare/common_comparison_category.h",
       "__compare/compare_three_way_result.h",
       "__compare/ordering.h",