Fix -Wconversion warning

[Tobi823]

Thanks for your great work and the great fmt library.

I have a small problem. When I cross-compile (with dockcross/linux-armv7 https://github.com/dockcross/dockcross to Raspberry Pi ARMv7) my source code with your fmt-library, I receive this warning:

/work/build_armv7/_deps/fmt-src/include/fmt/chrono.h:1217:37: warning: conversion from 'std::make_unsigned<long long int>::type' {aka 'long long unsigned int'} to 'size_t' {aka 'unsigned int'} may change value [-Wconversion]
 1217 |       copy2(buf, digits2(to_unsigned(year / 100)));
      |                          ~~~~~~~~~~~^~~~~~~~~~~~

Not using the -Wconversion flag is not an option because I want to use the flag to detect as many errors as possible.
And I'm not able to configure CMake, CPMAddPackage and target_include_directories+SYSTEM correctly to ignore warnings from your library 🙈.

My solution (adding const_cast<size_t>) works for me but I don't know if this is a clean solution. I'm not a C++ expert.

[vitaut]

Thanks for the PR.