[gn build] Port 3903438

Author: llvmgnsyncbot

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

@@ -92,6 +92,7 @@ if (current_toolchain == default_toolchain) {
       "__algorithm/find_if.h",
       "__algorithm/find_if_not.h",
       "__algorithm/find_segment_if.h",
+      "__algorithm/fold.h",
       "__algorithm/for_each.h",
       "__algorithm/for_each_n.h",
       "__algorithm/for_each_segment.h",