Add logger to spdlog's registry

[foresto]

As discussed here, mtxclient should register its logger with spdlog in order for Nheko's per-component log settings to work on the mtx component.

A quick search of today's mtxclient code doesn't yield any hits for spdlog::register_logger(), and nheko -l off,mtx=trace doesn't produce any output, so I guess it still needs doing.