Removed a compile warning
Removed a compile warning


Former-commit-id: b833b19a87345df91e72cdf15084011f15176aae
shuheik/dynet,clab/cnn,danielhers/dynet,clab/cnn,danielhers/dynet,xunzhang/dynet,danielhers/dynet,xunzhang/dynet,xunzhang/dynet,chunyang-wen/dynet,shuheik/dynet,chunyang-wen/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,clab/cnn,shuheik/dynet,chunyang-wen/dynet,clab/dynet,clab/dynet,clab/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,danielhers/dynet,shuheik/dynet,xunzhang/dynet,clab/dynet,clab/dynet,chunyang-wen/dynet,shuheik/dynet