Add store hints for OMEMO encrypted messages

Store hints are required has some server might discard messages without
body. Here we ensure OMEMO messages are stored on server and delivered
to client when they connect back.

It's really important since it avoid libsignal to desynchronize
counters.

src/xmpp/message.c

@@ -404,6 +404,8 @@ message_send_chat_omemo(const char *const jid, uint32_t sid, GList *keys,
         stanza_attach_state(ctx, message, state);
     }
 
+    stanza_attach_hints_store(ctx, message);
+
     if (request_receipt) {
         stanza_attach_receipt_request(ctx, message);
     }