fix sodium constant type (size_t instead of ulong_long)

It seems like libsodium exposes all constants as `size_t` (I checked at least the files `src/libsodium/crypto_box/curve25519xsalsa20poly1305/box_curve25519xsalsa20poly1305_api.c` and `src/libsodium/crypto_generichash/blake2/generichash_blake2_api.c` of libsodium). So this FFI declaration seems wrong and is probably the reason for issue #112.

I haven't actually tested this. @patte Could you test this on your Raspberry Pi?

@tarcieri Does this sound plausible? I don't know a lot about C.