Fix test-interface compilation error for MSVC x64

[OronDF343]

Simply removing redundant % to fix the following warnings treated as error:

..\..\tools\test.c(329): error C2220: warning treated as error - no 'object' file generated
..\..\tools\test.c(329): warning C4477: 'printf' : format string '%d' requires an argument of type 'int', but variadic argument 1 has type 'const size_t'
..\..\tools\test.c(329): note: consider using '%zd' in the format string
..\..\tools\test.c(329): warning C4477: 'printf' : format string '%d' requires an argument of type 'int', but variadic argument 2 has type 'const size_t'
..\..\tools\test.c(329): note: consider using '%zd' in the format string

[jonsneyers]

Thanks for the contribution!

Sorry for being so negligent/slow. The FLIF codec has been superseded by the JPEG XL codec, and I was too busy with jxl to keep an eye on the flif repo. Please consider moving any project that is using FLIF to use libjxl instead.