fix r9713 for #36565

Authorised by: AcydBurn

git-svn-id: file:///svn/phpbb/branches/phpBB-3_0_0@9931 89ea8834-ac86-4346-8a33-228a782c2dd0

phpBB/install/schemas/mysql_40_schema.sql

@@ -459,6 +459,7 @@ CREATE TABLE phpbb_posts (
 	KEY poster_ip (poster_ip),
 	KEY poster_id (poster_id),
 	KEY post_approved (post_approved),
+	KEY post_username (post_username(255)),
 	KEY tid_post_time (topic_id, post_time)
 );