[gn build] Port cf50936

Author: llvmgnsyncbot

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

@@ -328,6 +328,7 @@ if (current_toolchain == default_toolchain) {
       "__chrono/exception.h",
       "__chrono/file_clock.h",
       "__chrono/formatter.h",
+      "__chrono/gps_clock.h",
       "__chrono/hh_mm_ss.h",
       "__chrono/high_resolution_clock.h",
       "__chrono/leap_second.h",