wallet: make BackupWallet() const

src/wallet/wallet.cpp

@@ -4004,7 +4004,7 @@ void CWallet::postInitProcess()
     chain().requestMempoolTransactions(*this);
 }
 
-bool CWallet::BackupWallet(const std::string& strDest)
+bool CWallet::BackupWallet(const std::string& strDest) const
 {
     return database->Backup(strDest);
 }

src/wallet/wallet.h

@@ -1098,7 +1098,7 @@ public:
      */
     void postInitProcess();
 
-    bool BackupWallet(const std::string& strDest);
+    bool BackupWallet(const std::string& strDest) const;
 
     /* Returns true if HD is enabled */
     bool IsHDEnabled() const;