[gn build] Port 77ad77c

Author: llvmgnsyncbot

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

@@ -320,6 +320,7 @@ if (current_toolchain == default_toolchain) {
       "__format/formatter_floating_point.h",
       "__format/formatter_integer.h",
       "__format/formatter_integral.h",
+      "__format/formatter_output.h",
       "__format/formatter_pointer.h",
       "__format/formatter_string.h",
       "__format/parser_std_format_spec.h",