tests/ffi: Added c-flag to find libffi's .h files.
tests/ffi: Added c-flag to find libffi's .h files.

This will be needed for any project using CFunction.  libffi's refusal to put
its .h files in a standard location (eg. /usr/include) is a real problem for
us.

stevefolta/trylon,stevefolta/trylon,stevefolta/trylon,stevefolta/trylon,stevefolta/trylon