[gn build] Manually port 6f2d8cc

Author: aeubanks

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

@@ -352,6 +352,7 @@ if (current_toolchain == default_toolchain) {
       "__chrono/formatter.h",
       "__chrono/hh_mm_ss.h",
       "__chrono/high_resolution_clock.h",
+      "__chrono/leap_second.h",
       "__chrono/literals.h",
       "__chrono/month.h",
       "__chrono/month_weekday.h",

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

@@ -315,6 +315,7 @@ if (libcxx_enable_experimental) {
     sources = [ "experimental/keep.cpp" ]
     if (libcxx_enable_filesystem && libcxx_enable_time_zone_database) {
       sources += [
+        "include/tzdb/leap_second_private.h",
         "include/tzdb/time_zone_link_private.h",
         "include/tzdb/time_zone_private.h",
         "include/tzdb/types_private.h",