NbtkTooltip: fix compiler warning
NbtkTooltip: fix compiler warning

clutter-project/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,jonnylamb/mx