fix: avoid an implicit conversion using size_t

The number of bits is used to express the size of a buffer. Using an
`int` causes an implicit conversion warning, let's use a `size_t` which
is the right type for the job.

include/fmt/format.h

@@ -2018,7 +2018,7 @@ FMT_CONSTEXPR FMT_INLINE auto write_int(OutputIt out, write_int_arg<T> arg,
                                         const format_specs& specs) -> OutputIt {
   static_assert(std::is_same<T, uint32_or_64_or_128_t<T>>::value, "");
 
-  constexpr int buffer_size = num_bits<T>();
+  constexpr size_t buffer_size = num_bits<T>();
   char buffer[buffer_size];
   if (is_constant_evaluated()) fill_n(buffer, buffer_size, '\0');
   const char* begin = nullptr;