RmxAudioPlayer.swift: minor warnings

[mustafa0x]

RmxAudioPlayer.swift:556:66: string interpolation produces a debug description for an optional value; did you mean to make this explicit?
                print("observe change currentItem: lastTrackId \(self.lastTrackId) playerItem: \(playerItem?.trackId)")
                                                                 ^~~~~~~~~~~~~~~~

RmxAudioPlayer.swift:556:98: string interpolation produces a debug description for an optional value; did you mean to make this explicit?
                print("observe change currentItem: lastTrackId \(self.lastTrackId) playerItem: \(playerItem?.trackId)")
                                                                 ~~~~~^~~~~~~~~~~

RmxAudioPlayer.swift:712:53: string interpolation produces a debug description for an optional value; did you mean to make this explicit?
            print("Queue changed current item to: \(playerItem.trackId)")
                                                                                                 ~~~~~~~~~~~~^~~~~~~

RmxAudioPlayer.swift:783:30: string interpolation produces a debug description for an optional value; did you mean to make this explicit?
                    print("\(playerItem.error)")
                                                    ~~~~~~~~~~~^~~~~~~