fix mysql (guid) schema bug

schema/ttrss_schema_mysql.sql

@@ -32,7 +32,7 @@ create table ttrss_entries (id integer not null primary key auto_increment,
 	feed_id integer not null references ttrss_feeds(id) ON DELETE CASCADE, 
 	updated datetime not null, 
 	title text not null, 
-	guid text not null unique, 
+	guid varchar(255) not null unique, 
 	link text not null, 
 	content text not null,
 	content_hash varchar(250) not null,