[Qt] Change RBF checkbox to reflect -walletrbf setting

Before this commit, the checkbox would always start off unchecked. After this
commit it will respect the -walletrbf setting (which is currently false by
default).

src/qt/walletmodel.cpp

@@ -706,3 +706,8 @@ int WalletModel::getDefaultConfirmTarget() const
 {
     return nTxConfirmTarget;
 }
+
+bool WalletModel::getDefaultWalletRbf() const
+{
+    return fWalletRbf;
+}