Fix printf for when uint64_t != unsigned long long int

Fix printf for when uint64_t != unsigned long long int

Thanks to Michal Sojka <sojkam1@fel.cvut.cz> for pointing out the
correct fix, which I verified in the freely-available WG14/N1124 draft
(from the C99 working group) which is available here:

http://www.open-std.org/JTC1/SC22/wg14/www/docs/n1124.pdf