I've also encountered this twice so far in working on OpenSSL bindings. Some OpenSSL APIs accept a time_t argument. It seems perhaps I can work around this by writing typedef int time_t; but it seems like typedef ... time_t; (or some other spelling to tell cffi to ask the compiler for the size) would be better.
Another example that arose that is actually OpenSSL specific is the BN_ULONG type. I worked around this in the same way but I'm more afraid to do this because I don't know what future versions of OpenSSL will do to BN_ULONG (I also don't know if typedef int BN_ULONG; is actually correct or not given the actual type definition is unsigned and can possibly be any of int, long, or even long long depending on platform).