Fix MSVC warning in std::chrono::time_point formatter

[hmbj]

The condition is constexpr causing MSVC level 4 warning:
warning C4127: conditional expression is constant

Made condition constexpr to eliminate the warning.

Code to reproduce the issue:

#include <fmt/chrono.h>
#include <fmt/xchar.h>
int main()
{
    fmt::format(L"{}", std::chrono::system_clock::time_point());
}

CMakeLists.txt:

cmake_minimum_required(VERSION 3.14)
project(mytest LANGUAGES CXX)

include(FetchContent)
FetchContent_Declare(fmt
  GIT_REPOSITORY  https://github.com/fmtlib/fmt.git
  GIT_TAG         10.0.0
)
FetchContent_MakeAvailable(fmt)

add_compile_options("$<$<CXX_COMPILER_ID:MSVC>:/W4>")
add_executable(mytest main.cpp)
target_compile_features(mytest PRIVATE cxx_std_17)
target_link_libraries(mytest PRIVATE fmt::fmt-header-only)

[vitaut]

Thanks