Include fmtlib's `xchar` header to include `fmt::basic_format_string`.
Otherwise, compilation with an external fmtlib 11.1.1 fails with
```
In file included from include/spdlog/spdlog.h:12:
include/spdlog/common.h:369:49: error: no template named 'basic_format_string' in namespace 'fmt'; did you mean 'std::basic_format_string'?
369 | inline fmt::basic_string_view<T> to_string_view(fmt::basic_format_string<T, Args...> fmt) {
| ^~~~~
```
Signed-off-by: Christian Blichmann <cblichmann@google.com>
The meaning of using quotes to #include is implementation defined, so it
may or not may be what we want. At least POSIX
(https://pubs.opengroup.org/onlinepubs/9699919799/utilities/c99.html)
says: "headers whose names are enclosed in double-quotes ( "" ) shall be
searched for first in the directory of the file with the #include line",
so not what we want since "spdlog" ends up twice in the path.
Paths pointing to the root of the library where replaced for ones relatives to each file.
For example, inside /include/spdlog/details/file_helper.h:
This will look for os.h in /include/spdlog/details/spdlog/details/ which doesn't exists.
replaced with:
