ns3-dce: Add return-statement to register_shrinker

Otherwise, certain compiler will complain that the function returns void in a
function returning a non-void value.

Fixes: 1e9aa348d8b3 (adapt new API around v3.11-rc7)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Merge pull request #2 from cpaasch/sim-ns3-dev-branch

ns3-dce: Add return-statement to register_shrinker