[NbtkBoxLayout] fix compiler warning
[NbtkBoxLayout] fix compiler warning

pick() prototype colour parameter is const

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