[gn build] Port a749e68

Author: llvmgnsyncbot

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

@@ -1038,6 +1038,7 @@ if (current_toolchain == default_toolchain) {
       "__format/enable_insertable.h",
       "__format/escaped_output_table.h",
       "__format/extended_grapheme_cluster_table.h",
+      "__format/fmt_pair_like.h",
       "__format/format_arg.h",
       "__format/format_arg_store.h",
       "__format/format_args.h",