fix cpplint
fix cpplint

xchainer/graph_scope.h:13:  Constructors callable with one argument should be marked explicit.  [runtime/explicit] [5]

ktnyt/chainer,chainer/chainer,chainer/chainer,niboshi/chainer,hvy/chainer,okuta/chainer,niboshi/chainer,niboshi/chainer,hvy/chainer,wkentaro/chainer,jnishi/chainer,pfnet/chainer,okuta/chainer,niboshi/chainer,ktnyt/chainer,keisuke-umezawa/chainer,wkentaro/chainer,okuta/chainer,keisuke-umezawa/chainer,ktnyt/chainer,jnishi/chainer,hvy/chainer,hvy/chainer,chainer/chainer,keisuke-umezawa/chainer,tkerola/chainer,keisuke-umezawa/chainer,jnishi/chainer,okuta/chainer,wkentaro/chainer,chainer/chainer,jnishi/chainer,wkentaro/chainer,ktnyt/chainer