Fixed compile error on older versions of gcc
Fixed compile error on older versions of gcc


Former-commit-id: d7fa7d7150bfd213140f8c9cd09d5d2c000732ed
xunzhang/dynet,danielhers/dynet,xunzhang/dynet,clab/cnn,clab/dynet,shuheik/dynet,clab/dynet,danielhers/dynet,chunyang-wen/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,danielhers/dynet,xunzhang/dynet,danielhers/dynet,chunyang-wen/dynet,clab/cnn,shuheik/dynet,clab/dynet,clab/dynet,clab/cnn,shuheik/dynet,chunyang-wen/dynet,clab/dynet,shuheik/dynet,clab/dynet,chunyang-wen/dynet,danielhers/dynet,danielhers/dynet,xunzhang/dynet,shuheik/dynet,shuheik/dynet