hakyll.cabal: allow mtl-2.2
hakyll.cabal: allow mtl-2.2

Signed-off-by: Sergei Trofimovich <110fdadc76b6bce16977f568bdb4723eaed6fb3f@gentoo.org>

Minoru/hakyll,d12frosted/hakyll,oldmanmike/hakyll,beni55/hakyll,Javran/hakyll