Install xdg mimetype definition for OpenCL C

Summary:
Installing this mimetype ourselves to complete the
system mimetype db will help people for now.

Reviewers: #kdevelop, qi437103, aaronpuchert, kfunk

Reviewed By: #kdevelop, kfunk

Subscribers: nalvarez, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D5621