scan-tree: Support gitweb.description.

Use gitweb.description instead of description file to determine
description, if option is enabled.
diff --git a/cgit.h b/cgit.h
index 6ee6769..f4d0e52 100644
--- a/cgit.h
+++ b/cgit.h
@@ -199,6 +199,7 @@
 	int embedded;
 	int enable_filter_overrides;
 	int enable_gitweb_owner;
+	int enable_gitweb_desc;
 	int enable_http_clone;
 	int enable_index_links;
 	int enable_commit_graph;