bidi.css loading was removed by a merge somewhere

we merged bidi into stylesheets.css a while ago in master, but when merging 3.3 style hashing into master it reverted the stylesheet.css change but left the change to overall_header.html

https://github.com/phpbb/phpbb/commit/5f0b3c2141b43f78078ae519292c1e4007e97803#diff-33545b62885e557479ce54ec5a1954392008c0092953f608991ee8971160de58