In fmt master, basic_format_string has moved from base.h to xchar.h

[pratikpc]

Took place in this commit

This change causes compile time errors due to:-

1. Using SPDLOG_FMT_EXTERNAL and using fmt master
2. spdlog/common needing basic_format_string without SPDLOG_USE_STD_FORMAT

   spdlog/include/spdlog/common.h

   Lines 367 to 370 in e593f66

   	#ifndef SPDLOG_USE_STD_FORMAT
   	template <typename T, typename... Args>
   	inline fmt::basic_string_view<T> to_string_view(fmt::basic_format_string<T, Args...> fmt) {
   	return fmt;

3. xchar.h not being included due to absence of the specified definitions
   

   spdlog/include/spdlog/common.h

   Lines 56 to 58 in e593f66

   	#if defined(SPDLOG_WCHAR_FILENAMES) || defined(SPDLOG_WCHAR_TO_UTF8_SUPPORT)
   	#include <spdlog/fmt/xchar.h>
   	#endif

[pratikpc]

I solved it personally for myself by removing the ifdefs around the headers

[tt4g]

This is a change that has not yet been released, so we will figure out how to address it after its release.