Remove directories from include paths

Previously this was a relative path which worked in the GHC tree, but
failed elsewhere. This caused trouble for out-of-tree users as well as
Hadrian, which wants to move build artifacts out of the working
directory. Fixes #8040.

Test Plan: Validate

Reviewers: thomie, austin, snowleopard, hvr

Reviewed By: snowleopard, hvr

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2530

GHC Trac Issues: #8040