Call dbus dependent code only if dbus imported
Call dbus dependent code only if dbus imported

Addresses another symptom of mopidy/mopidy#1335

jodal/mopidy,adamcik/mopidy,vrs01/mopidy,vrs01/mopidy,tkem/mopidy,ZenithDK/mopidy,vrs01/mopidy,kingosticks/mopidy,mokieyue/mopidy,jcass77/mopidy,vrs01/mopidy,jodal/mopidy,kingosticks/mopidy,jodal/mopidy,tkem/mopidy,tkem/mopidy,tkem/mopidy,mopidy/mopidy,ZenithDK/mopidy,adamcik/mopidy,mopidy/mopidy,jcass77/mopidy,mokieyue/mopidy,ZenithDK/mopidy,adamcik/mopidy,jcass77/mopidy,mopidy/mopidy,kingosticks/mopidy,mokieyue/mopidy,ZenithDK/mopidy,mokieyue/mopidy