ghci: Ensure that system libffi include path is searched

Previously hsc2hs failed when building against a system FFI.
19 jobs for !768 with wip/system-libffi in 5 minutes and 53 seconds (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Lint
passed #72387
lint
ghc-linters

00:01:16

passed #72388
lint
lint-submods-mr

00:00:11

 
  Build
canceled #72390
x86_64-linux
hadrian-ghc-in-ghci

00:04:30

canceled #72392
x86_64-linux allowed to fail
release-x86_64-linux-deb9-dwarf

00:04:30

canceled #72389
x86_64-linux allowed to fail
validate-x86_64-linux-deb8-hadrian

00:04:31

canceled #72391
x86_64-linux
validate-x86_64-linux-deb9-debug

00:04:30

 
  Full Build
canceled #72394
aarch64-linux allowed to fail
validate-aarch64-linux-deb9

canceled #72395
x86_64-linux
validate-i386-linux-deb9

canceled #72393
x86_64-darwin
validate-x86_64-darwin

canceled #72396
x86_64-linux
validate-x86_64-linux-deb9

canceled #72398
x86_64-linux
validate-x86_64-linux-deb9-integer-simple

canceled #72397
x86_64-linux
validate-x86_64-linux-deb9-llvm

canceled #72399
x86_64-linux
validate-x86_64-linux-deb9-unreg

canceled #72400
x86_64-linux
validate-x86_64-linux-fedora27

canceled #72401
x86_64-windows allowed to fail
validate-x86_64-windows

 
  Cleanup
canceled #72403
x86_64-darwin
cleanup-darwin

canceled #72402
x86_64-windows
cleanup-windows

 
  Packaging
canceled #72404
x86_64-linux allowed to fail
doc-tarball

 
  Hackage
canceled #72405
x86_64-linux allowed to fail manual
hackage