[util] Remove unneeded virtual

clang warning:

../../util/options.hh:72:13: warning: destination for this 'memset' call is a pointer to dynamic class
      'option_parser_t'; vtable pointer will be overwritten [-Wdynamic-class-memaccess]
    memset (this, 0, sizeof (*this));
    ~~~~~~  ^
../../util/options.hh:72:13: note: explicitly cast the pointer to silence this warning
    memset (this, 0, sizeof (*this));
            ^
            (void*)

util/options.hh

@@ -76,7 +76,7 @@ struct option_parser_t
 
     add_main_options ();
   }
-  virtual ~option_parser_t (void)
+  ~option_parser_t (void)
   {
     g_option_context_free (context);
     g_ptr_array_foreach (to_free, (GFunc) g_free, nullptr);