Platform: correct posix_file_actions_init on Android

Adjust the API annotations to get this imported into Swift properly. The
outer pointer may be `nullptr`, but the inner pointer may not.

Author: compnerd

stdlib/public/Platform/spawn.apinotes

@@ -11,3 +11,7 @@ Functions:
     Type: "const posix_spawn_file_actions_t _Nonnull * _Nullable"
   - Position: 3
     Type: "const posix_spawnattr_t _Nonnull * _Nullable"
+- Name: posix_spawn_file_actions_init
+  Parameters:
+  - Position: 0
+    Type: "posix_spawn_file_actions_t _Nullable * _Nonnull"