[gn build] Port 9d16cbc

Author: llvmgnsyncbot

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

@@ -460,11 +460,17 @@ if (current_toolchain == default_toolchain) {
       "__functional/unwrap_ref.h",
       "__functional/weak_result_type.h",
       "__fwd/array.h",
+      "__fwd/fstream.h",
       "__fwd/get.h",
       "__fwd/hash.h",
+      "__fwd/ios.h",
+      "__fwd/istream.h",
       "__fwd/memory_resource.h",
+      "__fwd/ostream.h",
       "__fwd/pair.h",
       "__fwd/span.h",
+      "__fwd/sstream.h",
+      "__fwd/streambuf.h",
       "__fwd/string.h",
       "__fwd/string_view.h",
       "__fwd/subrange.h",