[gn build] Port aca829d

Author: llvmgnsyncbot

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

@@ -342,6 +342,7 @@ if (current_toolchain == default_toolchain) {
       "__chrono/steady_clock.h",
       "__chrono/sys_info.h",
       "__chrono/system_clock.h",
+      "__chrono/tai_clock.h",
       "__chrono/time_point.h",
       "__chrono/time_zone.h",
       "__chrono/time_zone_link.h",