Refactored uses of shared_ptr::unique as it's deprecated.

src/qt/deniabilitydialog.cpp

@@ -1451,7 +1451,7 @@ std::shared_ptr<CTxDestination> DeniabilityDialog::reserveDestination(interfaces
     Assert(m_model && m_model->getEncryptionStatus() != WalletModel::Locked);
 
     for (const auto& destination : m_reservedDestinations) {
-        if (destination.unique()) {
+        if (destination.use_count() == 1) {
             return destination;
         }
     }