Fix build on NetBSD, which provides posix_spawn_file_actions_addchdir()
but not the _np() variant.

Upstream PR: https://github.com/sheredom/subprocess.h/pull/102
Applied locally by scripts/sync_vendor.py until it is merged upstream.

diff --git a/subprocess.h b/subprocess.h
index 5e809023a4..74a4e006c7 100644
--- a/subprocess.h
+++ b/subprocess.h
@@ -1205,7 +1205,7 @@ cleanup:
 
   // Set working directory
   if (process_cwd) {
-#if defined(__APPLE__) && MAC_OS_X_VERSION_MIN_REQUIRED >= 260000
+#if defined(__NetBSD__) || (defined(__APPLE__) && MAC_OS_X_VERSION_MIN_REQUIRED >= 260000)
     posix_error = posix_spawn_file_actions_addchdir(&actions, process_cwd);
 #else
 #if defined(__APPLE__) && defined(__clang__)