fix show_description.c to support trema-edge

Build:
  cd /somewhere/apps/show_description
  make TREMA_CONFIG=/somewhere/trema-edge/trema-config
Example:
  cd /somewhere/trema-edge
  ./trema run src/examples/learning_switch/learning-switch.rb -c src/examples/learning_switch/sample.conf -d
  ./trema run /somewhere/apps/show_description/show_description