add SendCoinsEntry::updateLabel() function

- makes the code for updating a label reusable
- no functional changes

src/qt/sendcoinsentry.h

@@ -58,6 +58,8 @@ private:
     SendCoinsRecipient recipient;
     Ui::SendCoinsEntry *ui;
     WalletModel *model;
+
+    bool updateLabel(const QString &address);
 };
 
 #endif // SENDCOINSENTRY_H