Make nbd-proxy header build in all cases

We very intentionally don't do this "only include header if option is
enabled" thing to make sure that compile issues are seen across all
builds.

Tested: Code compiles.  Header changes only.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I749aed62ed6cd73690f3d89d75df65bec77562c2