cpp: Use #pragma once instead of #ifndef guards

This both says what we mean and silences a bunch of spurious CPP linting
warnings. This pragma is supported by all CPP implementations which we
support.

Reviewers: austin, erikd, simonmar, hvr

Reviewed By: simonmar

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3482