possible incorrectness in libffi include dir assignment, doesn't seem impactful #61

This looks like an obvious mistake but I don't think it matters because the pkgconfig files are correct and provide the path ultimately:

            if self.active_module(LibFFI):
                yield f"LIBFFI_INCLUDEDIR={self.deps_lib}"

I'll refrain from opening a PR because I'm not sure... it's possible that this is necessary due to libffi nuances and just looks wrong, but there's no comment or commit message to indicate if thats the case.