[gn build] Port 34b2e93

Author: llvmgnsyncbot

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

@@ -1155,6 +1155,7 @@ if (current_toolchain == default_toolchain) {
       "__iterator/ostreambuf_iterator.h",
       "__iterator/permutable.h",
       "__iterator/prev.h",
+      "__iterator/product_iterator.h",
       "__iterator/projected.h",
       "__iterator/ranges_iterator_traits.h",
       "__iterator/readable_traits.h",