wallet/db: make Backup() const

This method is the to-disk equivalent of serialize methods which are also const.

src/wallet/db.cpp

@@ -850,7 +850,7 @@ bool BerkeleyDatabase::Rewrite(const char* pszSkip)
     return BerkeleyBatch::Rewrite(*this, pszSkip);
 }
 
-bool BerkeleyDatabase::Backup(const std::string& strDest)
+bool BerkeleyDatabase::Backup(const std::string& strDest) const
 {
     if (IsDummy()) {
         return false;

src/wallet/db.h

@@ -157,7 +157,7 @@ public:
 
     /** Back up the entire database to a file.
      */
-    bool Backup(const std::string& strDest);
+    bool Backup(const std::string& strDest) const;
 
     /** Make sure all changes are flushed to disk.
      */