xs: update libffi
Changes: update
minor clean-up
v0.18

 - Add long and unsigned long ffi types (GH#7) (thanks, @calid!)
 - Add support for 64 bit ffi integer types (GH#9) (thanks, @calid!)
 - Fix invalid memory accesses in struct tests (should fix many failures)
 - Update libffi to 2f5b7ce