[gn build] Port bf63b15

Author: llvmgnsyncbot

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

@@ -153,6 +153,7 @@ if (current_toolchain == default_toolchain) {
       "__algorithm/pstl_backends/cpu_backends/fill.h",
       "__algorithm/pstl_backends/cpu_backends/find_if.h",
       "__algorithm/pstl_backends/cpu_backends/for_each.h",
+      "__algorithm/pstl_backends/cpu_backends/merge.h",
       "__algorithm/pstl_backends/cpu_backends/serial.h",
       "__algorithm/pstl_backends/cpu_backends/thread.h",
       "__algorithm/pstl_backends/cpu_backends/transform.h",
@@ -161,6 +162,7 @@ if (current_toolchain == default_toolchain) {
       "__algorithm/pstl_find.h",
       "__algorithm/pstl_for_each.h",
       "__algorithm/pstl_frontend_dispatch.h",
+      "__algorithm/pstl_merge.h",
       "__algorithm/pstl_transform.h",
       "__algorithm/push_heap.h",
       "__algorithm/ranges_adjacent_find.h",