Fix missing update of BPM display

The problem was a missing redraw after changing the label. This led to a
stucked display, when the tempo dial is used. Now a redraw is added after
the label is changed.

src/gmastertrack.cxx

@@ -331,6 +331,7 @@ void GMasterTrack::setBpm( float b )
 	std::stringstream s;
 	s << round(bpm*10)/10;
 	tempoDial.copy_label( s.str().c_str() );
+	tempoDial.redraw();
 }
 
 void GMasterTrack::setInputVol(float f)