Add -Wextra to compile options (#530)
Add -Wextra to compile options (#530)


olofer/LightGBM,olofer/LightGBM,Laurae2/LightGBM,huanzhang12/LightGBM,wxchan/LightGBM,henry0312/LightGBM,huanzhang12/LightGBM,microsoft/LightGBM,microsoft/LightGBM,huanzhang12/LightGBM,henry0312/LightGBM,Allardvm/LightGBM,wxchan/LightGBM,Allardvm/LightGBM,henry0312/LightGBM,Allardvm/LightGBM,Laurae2/LightGBM,Allardvm/LightGBM,microsoft/LightGBM,huanzhang12/LightGBM,huanzhang12/LightGBM,olofer/LightGBM,Laurae2/LightGBM,olofer/LightGBM,wxchan/LightGBM,olofer/LightGBM,wxchan/LightGBM,wxchan/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,Allardvm/LightGBM,Laurae2/LightGBM,Laurae2/LightGBM,henry0312/LightGBM