MSVC: Conditional expression is constant warning when formatting `tm` with unicode disabled

[edo9300]

When formatting a tm struct with unicode support disabled, MSVC outputs the warning C4127 on this row

fmt/include/fmt/chrono.h

Line 367 in 093b39c

if (detail::use_utf8 && loc != get_classic_locale()) {

this is a sample repro scenario

#define FMT_UNICODE 0
#include <fmt/core.h>
#include <fmt/chrono.h>
#include <ctime>

int main() {
	auto now = std::time(nullptr);
	fmt::format("[{:%Y-%m-%d %H:%M:%S}]", fmt::localtime(now));
	return 0;
}

(compile with /W4)

[vitaut]

We don't actively look into level 4 warnings since they are mostly noise but a PR to fix it would be welcome.