wallet_rpc_server: add missing background check

Reported by ro1m

src/wallet/wallet_rpc_server.cpp

@@ -2786,6 +2786,7 @@ namespace tools
   bool wallet_rpc_server::on_get_spend_proof(const wallet_rpc::COMMAND_RPC_GET_SPEND_PROOF::request& req, wallet_rpc::COMMAND_RPC_GET_SPEND_PROOF::response& res, epee::json_rpc::error& er, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);
+    CHECK_IF_BACKGROUND_SYNCING();
 
     crypto::hash txid;
     if (!epee::string_tools::hex_to_pod(req.txid, txid))