libdmtx: Use the new GitHub mirror

Summary:
The old SourceForge mirror has been removed, update to the new GitHub mirror to fix the downloads.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

Reviewers: vkrause

Reviewed By: vkrause

Differential Revision: https://phabricator.kde.org/D18197