Add a virtual destructor to DB_

This removes warning about deleting an object with a non-virtual
destructor in the base class.

convert_db.h

@@ -39,6 +39,7 @@ class DB_ {
 		virtual bool fill_database(DB_ * old_database);
 		virtual void close_db();
 		virtual DBC * get_database();
+    virtual ~DB_();
 };
 /*
  * Libdb class needs only open and read data from libdb database

db.cc

@@ -21,6 +21,10 @@ DBC * DB_::get_database() {
 	return NULL;
 }
 void DB_::close_db(){}
+
+DB_::~DB_() {
+}
+
 bool Libdb::connect_database(std::string path){
 	DB * db;
 	int status;