finagle: update build.sbt to ensure tcnative binary is available at runtime
finagle: update build.sbt to ensure tcnative binary is available at runtime

Problem/Solution

Our finagle-netty4 builds are failing via SBT due
to missing class definitions. Let's ensure that the
tcnative boringssl static binary is available for
tests at runtime.

JIRA Issues: CSL-10352

Differential Revision: https://phabricator.twitter.biz/D577066

twitter/finagle,twitter/finagle,twitter/finagle,twitter/finagle,twitter/finagle