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

yoavg/cnn,mitchellstern/dynet,Peilun-Li/dynet,Peilun-Li/dynet,kamigaito/cnn,dhgarrette/cnn,Abasyoni/dynet,kamigaito/cnn,cherryc/dynet,mitchellstern/dynet,Peilun-Li/dynet,dhgarrette/cnn,Abasyoni/dynet,dhgarrette/cnn,cherryc/dynet,cherryc/dynet,yoavg/cnn,Abasyoni/dynet