Hello Guix,
This fixes bug #37380 for SDDM users.

Today I was setting up my system following [0] to use realtime 
scheduling with JACK and SuperCollider. After a lot of failures and a 
lot of help on IRC from rekado I found out that it wasn't me it was just 
a bug.

The bug is the same as [1] and my solution is the same as [2] but for 
SDDM. I tested my patch with the attached system configuration 
(shamelessly copied from [3]).

Thanks,

Giacomo

[0]: 
https://guix.gnu.org/manual/en/html_node/Base-Services.html#index-realtime
[1]: https://issues.guix.gnu.org/issue/37380
[2]: https://issues.guix.gnu.org/issue/37405
[3]: https://lists.gnu.org/archive/html/help-guix/2019-09/msg00065.html