Add default value for flood_cache

The original code left flood_cache undefined, leading to errors in newer versions of PHP. See https://github.com/vichan-devel/vichan/issues/525

inc/config.php

@@ -466,7 +466,9 @@
 	// outside the current board. This means that if you have a special flood condition for a specific board
 	// (contained in a board configuration file) which has a flood-time greater than any of those in the
 	// global configuration, you need to set the following variable to the maximum flood-time condition value.
+	// Set to -1 to disable.
 	// $config['flood_cache'] = 60 * 60 * 24; // 24 hours
+	$config['flood_cache'] = -1;
 
 /*
  * ====================