Fix fprintf warning to reflect changes in official KeccakCodePackage

Quick fix to make source match official KeccakCodePackage and avoid future problems. See here:

https://github.com/gvanas/KeccakCodePackage/commit/e11a2c9d9acb56ad9266c24c6e5d28835d386511#diff-7c0a0e0b667a8e262ec363891090f620