Update lcm_vector_gen.py to use pragma-once.
Update lcm_vector_gen.py to use pragma-once.

billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,sheim/drake,billhoffman/drake,billhoffman/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake