fix warning on strict compilers
fix warning on strict compilers

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