systemd: Add conflicts with pipewire-media-session

This allows to run WirePlumber preferably when pipewire-media-session is
also installed.
2 jobs for systemd-conflicts in 1 minute and 27 seconds
Status Job ID Name Coverage
  Container
passed container_fedora #9889395

00:00:47

 
  Build
passed build_on_fedora #9889396

00:00:39