Fix mimetype file installation (needs ECM & KDEInstallDirs)

Summary:
find_package( SharedMimeInfo) relies on some FindSharedMimeInfo.cmake file
as sharedmimeinfo does not come itself with cmake config files.
ECM provides a FindSharedMimeInfo.cmake file.
Not sure about platforms without ECM, are there any where the additional
Geo mimetypes should be also registered?

Reviewers: rahn, nienhueser

Reviewed By: nienhueser

Projects: #marble

Differential Revision: https://phabricator.kde.org/D1505