[gn build] Port 0cd794d

Author: llvmgnsyncbot

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

@@ -347,6 +347,7 @@ if (current_toolchain == default_toolchain) {
       "__chrono/time_zone_link.h",
       "__chrono/tzdb.h",
       "__chrono/tzdb_list.h",
+      "__chrono/utc_clock.h",
       "__chrono/weekday.h",
       "__chrono/year.h",
       "__chrono/year_month.h",