Changed topic permastruct to use _bbp_topic_slug option instead of bbp_get_topic_slug()

This ensures that the /forums prefix is not added to the url twice if
the prefix is enabled for all content types

Author: alfhen

src/Bonnier/WP/BBPress/BetterPermalinks/Permalinks.php

@@ -47,7 +47,7 @@ public static function add_post_types_rewrite($postType, $args)
                     'index.php?forumnames=$matches[1]&name=$matches[2]&post_type='.$postType,
                     'top'
                 );
-                add_permastruct($postType, bbp_get_forum_slug()."%forumnames%".bbp_get_topic_slug()."/%postname%/", $args->rewrite);
+                add_permastruct($postType, bbp_get_forum_slug()."%forumnames%".get_option( '_bbp_topic_slug', 'topic' )."/%postname%/", $args->rewrite);
                 static::flush_rewrite_rules_if_needed();
                 break;
         }