aptcc: Link against -lutil for forkpty

Commit 223ae57d468fdcac451209a095047a07a5698212 in apt causes libapt-pkg
to no longer be linked against -lutil, which means that aptcc's
reference to forkpty is no longer satisfied.  An explicit use should
have an explicit link.

Resolves: https://bugs.launchpad.net/bugs/1368246