minor bugfix for cmake
minor bugfix for cmake

Summary: TSIA

Reviewed By: asaadaldien

Differential Revision: D4778069

fbshipit-source-id: 965bd7e00738ed508d5a9b0cae109b73ba1e9b62

bwasti/caffe2,sf-wind/caffe2,davinwang/caffe2,sf-wind/caffe2,davinwang/caffe2,xzturn/caffe2,davinwang/caffe2,Yangqing/caffe2,caffe2/caffe2,pietern/caffe2,xzturn/caffe2,pietern/caffe2,sf-wind/caffe2,bwasti/caffe2,bwasti/caffe2,pietern/caffe2,bwasti/caffe2,sf-wind/caffe2,davinwang/caffe2,xzturn/caffe2,Yangqing/caffe2,Yangqing/caffe2,Yangqing/caffe2,pietern/caffe2,sf-wind/caffe2,Yangqing/caffe2,xzturn/caffe2,davinwang/caffe2,xzturn/caffe2,pietern/caffe2,bwasti/caffe2