Fix -Wconversion warning (fmtlib#2724)

* - try fixing the warning "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]" by casting int to size_t (occurs when cross-compiling to Raspberry Pi ARMv7)

* - remove call to to_unsigned() because using static_cast<size_t> is enough

include/fmt/chrono.h

@@ -1214,7 +1214,7 @@ template <typename OutputIt, typename Char> class tm_writer {
     char buf[10];
     size_t offset = 0;
     if (year >= 0 && year < 10000) {
-      copy2(buf, digits2(to_unsigned(year / 100)));
+      copy2(buf, digits2(static_cast<size_t>(year / 100)));
     } else {
       offset = 4;
       write_year_extended(year);