cryptonote_basic: is_coinbase() take prefix

Useful for wallet business logic, which stores the prefixes in `m_transfers`, not the whole transaction.

src/cryptonote_basic/cryptonote_basic_impl.cpp

@@ -172,7 +172,7 @@ namespace cryptonote {
     return tools::base58::encode_addr(integrated_address_prefix, t_serializable_object_to_blob(iadr));
   }
   //-----------------------------------------------------------------------
-  bool is_coinbase(const transaction& tx)
+  bool is_coinbase(const transaction_prefix& tx)
   {
     if(tx.vin.size() != 1)
       return false;