clang-tidy, qt: Fix modernize-use-default-member-init in headers

See https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-default-member-init.html

src/qt/walletmodeltransaction.h

@@ -41,7 +41,7 @@ public:
 private:
     QList<SendCoinsRecipient> recipients;
     CTransactionRef wtx;
-    CAmount fee;
+    CAmount fee{0};
 };
 
 #endif // BITCOIN_QT_WALLETMODELTRANSACTION_H