mpriscontrol plugin should ignore playerctld

playerctld (https://github.com/altdesktop/playerctl/issues/161) is a proxy daemon for the currently active player by playerctl, which facilitates managing mpris players, forwarding requests to the currently active/last active player, and sorting out troubles with selecting the correct player manually.
Unfortunately, it also creates an annoying issue with kdeconnect: when playing media on the phone, kdeconnect publishes the state to the computer through the mprisremote plugin - then, playerctld picks it up as active player, and registers its own mpris media player. As a result, the mpriscontrol plugin sees this as a running media player, and in turn, publishes the state back to the phone, essentially creating another media session on the phone, resulting in two notifications. As playerctld is _always_ only a proxy to another media player (or kdeconnect), it can safely be ignored, just like kdeconnect itself already is. This commit adds an if check doing exactly that.