Add missing xchar.h header for fmt > 11.0.2

[ahoarau]

Using fmt main branch, after the 11.0.2 release, fmt::basic_format_string is defined in xchar.h.

Otherwise compile error:

spdlog/common.h:369:49: error: no template named 'basic_format_string' in namespace 'fmt'; did you mean 'std::basic_format_string'?
[build]   369 | inline fmt::basic_string_view<T> to_string_view(fmt::basic_format_string<T, Args...> fmt) {
[build]       |                                                 ^~~~~
[build] /usr/lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/format:128:12: note: 'std::basic_format_string' declared here
[build]   128 |     struct basic_format_string
[build]       |            ^
[build] 1 error generated.

[gabime]

Something is wrong. It is already defined here

spdlog/include/spdlog/fmt/xchar.h

Line 21 in 951c5b9

#include <fmt/xchar.h>

[ahoarau]

@gabime I just saw that.. Not the right patch for the issue. Investiguating.