update literate haskell mimetype

Summary:
According to the freedesktop mimetypes literate haskell uses it's own mimetype
with the name text/x-literate-haskell

Reviewers: vkrause, cullmann

Reviewed By: cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D18998