[gn build] Port adc7932

Author: llvmgnsyncbot

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

@@ -336,6 +336,7 @@ if (current_toolchain == default_toolchain) {
       "__chrono/gps_clock.h",
       "__chrono/hh_mm_ss.h",
       "__chrono/high_resolution_clock.h",
+      "__chrono/is_clock.h",
       "__chrono/leap_second.h",
       "__chrono/literals.h",
       "__chrono/local_info.h",