openflow-lwt: Update to work with openflow.0.2.0

Starting with the openflow.0.2.0 package, the lwt-specific code is
included in a separate subpackage. Since the controller relies on that
lwd code, this commit adds it as a dependency.
openflow-lwt: enable lwt in openflow build
Merge pull request #195 from frenetic-lang/openflow-lwt

Update openflow library dependencies