[ticket/12498] Adds height-property for IE8 in tweaks.css

PHPBB3-12498

This fix adds a new "height"-property for the IE8 to keep the aspect-ratio of the header-avatar correct. In the moment the avatar is horizontally streched and this makes the header look ugly. (see picture in tracker-attachment) 

https://tracker.phpbb.com/browse/PHPBB3-12498