Fix #15073 by suggesting UnboxedTuples in an error message

Under certain circumstances, `GeneralizedNewtypeDeriving`
can emit code which uses unboxed tuple types, but if `UnboxedTuples`
wasn't enabled, the error message that GHC gave didn't make it very
clear that it could be worked around by explicitly enabling the
extension. Easily fixed.

Test Plan: make test TEST=T15073

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: simonpj, thomie, carter

GHC Trac Issues: #15073

Differential Revision: https://phabricator.haskell.org/D4620