Have GetRecipientPools return a copy

Previously returned a const reference to a non-const object, which could then be
modified under the caller’s feet.

src/wallet/wallet_tx_builder.h

@@ -99,7 +99,7 @@ public:
         payments.push_back(payment);
     }
 
-    const std::set<OutputPool>& GetRecipientPools() const {
+    std::set<OutputPool> GetRecipientPools() const {
         return recipientPools;
     }