rts/package.conf.in: fix UNREG on --with-system-libffi when include-dir is passed explicitely
rts/package.conf.in: fix UNREG on --with-system-libffi when include-dir is passed explicitely

Issue #8748

Signed-off-by: Sergei Trofimovich <110fdadc76b6bce16977f568bdb4723eaed6fb3f@gentoo.org>
Signed-off-by: Austin Seipp <7ea35d812706d9213868749011af1ed4fa2f6aa0@well-typed.com>
(cherry picked from commit 2d0fa9aee78e5bfffb9a4580954825025a651be5)

lukexi/ghc-7.8-arm64,hferreiro/replay,lukexi/ghc-7.8-arm64,hferreiro/replay,lukexi/ghc-7.8-arm64,hferreiro/replay,lukexi/ghc-7.8-arm64,hferreiro/replay,hferreiro/replay,hferreiro/replay,lukexi/ghc-7.8-arm64,hferreiro/replay