Cast away const to avoid a warning when cleaning up const data.

When cleaning up the vcd_list make sure to remove the const
qualifier before freeing the ident. This is created with strdup.

vpi/sys_vcd.c

@@ -261,7 +261,7 @@ static PLI_INT32 finish_cb(p_cb_data cause)
 
       for (cur = vcd_list ;  cur ;  cur = next) {
 	    next = cur->next;
-	    free(cur->ident);
+	    free((char *)cur->ident);
 	    free(cur);
       }
       vcd_names_delete(&vcd_tab);