Ensure column matches %ld format in jimp_diagf

[mikmart]

.. on Windows %ld expects 32 bits but ptrdiff_t is 64 bits.

Fixes the following warning when compiled with MSVC:

C:\Users\mikko\git\jim\jimp.h(277): warning C4477: 'fprintf' : format string '%ld' requires an argument of type 'long', but variadic argument 3 has type '__int64'
C:\Users\mikko\git\jim\jimp.h(277): note: consider using '%lld' in the format string
C:\Users\mikko\git\jim\jimp.h(277): note: consider using '%Id' in the format string
C:\Users\mikko\git\jim\jimp.h(277): note: consider using '%I64d' in the format string

Discovered when compiling the B compiler, thought it'd be best to contribute upstream.