Fix a typo in libffi detection code
Fix a typo in libffi detection code

ocamllabs/ocaml-ctypes,sjfloat/ocaml-ctypes,whitequark/ocaml-ctypes,sjfloat/ocaml-ctypes,yallop/ocaml-ctypes,dsheets/ocaml-ctypes,dsheets/ocaml-ctypes,ocamllabs/ocaml-ctypes,whitequark/ocaml-ctypes,whitequark/ocaml-ctypes,dsheets/ocaml-ctypes,sjfloat/ocaml-ctypes,yallop/ocaml-ctypes