[gn build] Port 695138c

Author: llvmgnsyncbot

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

@@ -750,6 +750,7 @@ if (current_toolchain == default_toolchain) {
       "__system_error/system_error.h",
       "__thread/formatter.h",
       "__thread/id.h",
+      "__thread/jthread.h",
       "__thread/poll_with_backoff.h",
       "__thread/this_thread.h",
       "__thread/thread.h",