Fix failed build.
Fix failed build.

I apparently think I'm good enough to get away without testing my changes ("It's
simple! Surely it will work!") yet time and time again I'm shown wrong.  For
some reason I never learn.

stereomatchingkiss/mlpack,ranjan1990/mlpack,darcyliu/mlpack,erubboli/mlpack,bmswgnp/mlpack,ranjan1990/mlpack,chenmoshushi/mlpack,ajjl/mlpack,trungda/mlpack,minhpqn/mlpack,chenmoshushi/mlpack,Azizou/mlpack,thirdwing/mlpack,ersanliqiao/mlpack,bmswgnp/mlpack,Azizou/mlpack,BookChan/mlpack,palashahuja/mlpack,datachand/mlpack,darcyliu/mlpack,minhpqn/mlpack,stereomatchingkiss/mlpack,BookChan/mlpack,lezorich/mlpack,bmswgnp/mlpack,lezorich/mlpack,erubboli/mlpack,ajjl/mlpack,lezorich/mlpack,ersanliqiao/mlpack,palashahuja/mlpack,erubboli/mlpack,palashahuja/mlpack,thirdwing/mlpack,trungda/mlpack,theranger/mlpack,ersanliqiao/mlpack,trungda/mlpack,Azizou/mlpack,datachand/mlpack,darcyliu/mlpack,BookChan/mlpack,thirdwing/mlpack,ajjl/mlpack,ranjan1990/mlpack,minhpqn/mlpack,stereomatchingkiss/mlpack,datachand/mlpack,theranger/mlpack,theranger/mlpack,chenmoshushi/mlpack