Be sure book file is closed before we leave

Move closing of file in Book destructor. This
guarantees us against leaving the file open under
any case and simplifies also Book use.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/book.h

@@ -53,8 +53,8 @@ struct BookEntry {
 };
 
 class Book : private std::ifstream {
-
 public:
+  ~Book();
   void open(const std::string& fName);
   void close();
   const std::string file_name() const;