Make changes-client build on non-Linux machines.
Make changes-client build on non-Linux machines.

Summary:
changes-client failed to build on Mac previously because it couldn't find any source files under
adapter/lxc. Add a dummy file to make sure compilation succeeds. This effectively disables LXC on
non-Linux machines.

Test Plan: go build github.com/dropbox/changes-client

Reviewers: kylec

Reviewed By: kylec

Subscribers: mkedia, wwu

Differential Revision: https://tails.corp.dropbox.com/D107547

dropbox/changes-client,dropbox/changes-client