fix flow_dumper.c to support trema-edge

Build flow_dumper for trema-edge
  make TREMA_CONFIG=/somewhere/trema-edge/trema-config

How to use?
  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/flow_dumper/flow_dumper