Make SePolicy have Send

It's safe to send between threads, and I want to do so
in ostree-rs-ext to send to a tokio worker thread.

rust-bindings/rust/conf/ostree.toml

@@ -192,6 +192,7 @@ status = "generate"
 [[object]]
 name = "OSTree.SePolicy"
 status = "generate"
+concurrency = "send"
     [[object.function]]
     # [IGNORE] has an unused raw pointer parameter
     name = "fscreatecon_cleanup"