Recognize MIME type text/x-opencl-src as OpenCL

Summary:
We should still leave the other path, since not everybody might have the
newest shared-mime-info.

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8644