small compile fix for older compilers
small compile fix for older compilers

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